That dentist example has nothing to do with this example. That's a playful joke that is also a real compliment about someones smile.
But Molly's situation is a guy belittling her by joking and assuming that he, who is someone that doesn't even train as a runner (most likely, otherwise he would have known her), could probably beat her even though she is a world class professional and Olympic medalist.
That's why it's so cringe. That's why the post blew up and 51k people liked it and over 1600 people commented on it. Women are used to constantly being talked down for everything because some men just assume (and make asses out of themselves) that they're probably better than them without trying. It's invalidating. Not playful or flirty.
Cause the comment doesn't just exists in a vacuum, it exists in a world where female athletes only get 4% of the sports media coverage and 1% of the sports sponsorship dollars. So yeah hearing stuff like that isn't going to impress your bumble match. It's not just Molly, no female pro runner would be impressed with that ridiculous and offensive pick up line, wether said in person or on an app.
I used to play volleyball in high school and we were one of the top teams in the area. There was talk of having a coed team for a bit (thankfully it never happened) and all of the guys that wanted to join talked about it as if they were guaranteed a spot on the team. They've never played volleyball outside of a few days at gym class, and assumed that they were gonna effortlessly beat out the girls that played and trained year round for their entire lives. We had to point out why their logic was sexist for them to finally understand. But it did get better with most of them once they understood and started self reflecting on just how often the things they used to say to their female friends without thinking was hurtful.
Hopefully Molly shining light on this will open the eyes to young men and boys to think first before opening their mouths and putting down a girl/women with an off handed compliment/joke that is insulting to them. Cause most boys/men do honestly say that like that not even understanding that it's hurtful and sexist. And the good ones will learn to undo their behavior cause they want to.