Is it possible to gain any discernible level of fitness through cycling?

Walking can give you decent fitness but those people may not be able to run a 5K. Some of it is exercise specific. He may be underestimating his fitness or being modest also.

Durianrider the YouTube celebrity cyclist only does a weekly ParkRun, and he regularly runs it in 17 minutes something. Strava is full of hobby cyclists who only do the occasional run and yet still manage to keep a good pace.

your question is basically do the guys at the tour de france have any modicum of fitness.

please.

He also clocked a ~ 2:48 marathon off of 10 mpw for the year he did it in.

Anton Krupica came back to Leadville in 2021 for third place on a fraction of the mileage he did historically for ultras and mainly just cycled for it. And Jack Foster back in the Commonwealth games in the early 70s set a masters record in the marathon in 2:11 and his mileage was between 40-80 MPW mainly on the lower end, but he cycled a lot. Cycling is definitely a underused tool by most runners.

In 2010 I finished Leadville 100 MTB in less than 10 hours. 6 weeks later I ran a marathon with zero run training in 3:35 and shoes full of blood.

From meeting a lot of fairly dedicated hobby cyclists they can post good starting times such low 20 minute or high teens 5ks off no running specific training but they are much more prone to injuries as they haven't built up the high intensity impact durability.

25mpw cycling isn't a huge amount though. A commuter would do that amount in a few days

A single 25 mile ride done at a very casual pace is likely to build the same running fitness a leisurely two-hour walk would. That is to say, not much.
As others have noted, cycling can be used to build fitness especially when you consider that it’s possible to bike for multiple hours a day, every day. Please don’t take this a brag but in my twenties I accompanied a relative on a 12-mile long run (part of their marathon training) off zero running and ~150 miles/week of cycling. It was a slow jog (and pre-GPS days but likely 10+ minute miles I’m guessing) but other than some soreness in my legs afterwards it was fairly painless.

Depending on how flat of an area they cycle in, 25 miles might not get them much past 1 hour, which isn't that much. Cycling can definitely help you get fit -- if you're doing a lot of other things right or if you're injury prone. A runner doing 70mpw will benefit from 2-3 1hr easy rides/week. An injury prone runner doing 30-40mpw will benefit from 3-4 rides/week, including some faster riding. There's no substitute for running but cycling can add to your weekly volume.

As with any training, it depends on how it's done. Actual cyclist that have a solid training plan: they're fit but running often trashes their shins unless it's constantly included in their training

Fitness is exercise-specific to some degree. High-level cyclists have excellent aerobic capacity and very strong legs for cycling, but if they don't run at all they may not have the requisite strength/endurance in their running muscles. I used to run 70 mpw but a fast 5-mile walk would leave me with sore muscles, as even walking and running use different muscles.

That being said, 25 miles is a short bike ride for a serious road cyclist unless it is done at a very hard pace.

He is also on PEDS

One of the most egregious grifters in internet history.

cycling is exercise yes. But as an aging ex long time runner who cycles fairly regularly now ...I can say that running.......is hard. Its weird. I started when i was 5 so i didnt know the difference until i retired the running. Now in my 50s, Running is effing hard. lance Armstrong had some quote about how horrible his body was beat up (at the height of his powers) when he started training for marathon.

Running marathons is hard, and IMO usually pretty stupid if you value your health above some psychological need to prove yourself (I ran 6 while in college and wish I didn't). Modern shoes and smart training can extend enjoyable running almost indefinitely. Train accordingly.

Cycling and other "low impact" cross training activities don't prepare one well for "high impact" activities. People who don't run regularly will experience quad soreness when they start running.

It's also harder to "throttle back" running. Even jogging as slowly as possible is a big step up in effort from "not running". On a bike, unless there are significant hills, one can always go easier.

I'm having a hard time understanding how somebody who had three road bikes and "races people online" is only riding 25 miles on the weekend.

Regardless, if the guy wants to do Parkrun and feels the need to run/walk, that's not a reason to not show up. Everybody starts somewhere.

My favorite was when he accused Phil Gaimon of being on drugs after Phil wiped the floor with him.

It is a 5K. He should be able to run it. A problem might be going out too fast and suffering, but it is all of 5K.

I love Anton and was so stoked about his comeback but I think he might be a bit of an outlier with his volume. Dude was doing 4-500 mile weeks on the bike during some of the tours on top of 50-70 miles running, he’s an aerobic monster

And, how old is he?