Yeah if anything the last 5 years have shown us how impressive the old distance records were. Sure the record book has been refreshed (everything now except the mile and the 3000 which are honestly imminent) - but the playing field is totally different. I mean we all understood the value of metronomic, even paced running - I don't think we understood just how much of a benefit it was. Even if we did we had to rely on ourselves/another athlete to execute it and it proved absolutely impossible to do with the accuracy and duration of current technologies allowed in racing. Put it this way, had El G, Komen, KB been afforded the same conditions as the guys today, these records would have been markedly better than they were already. And of course the shorter the distance the "easier" it is in essence to get things right in that respect which is why many of those records may be approaching the plausible limits of human beings as we know them today. The 1500m is the only outlier there but it was saved by an outlier final 300m of the race - how that was possible? Draw your own conclusions.
As for a 1.48-1.49 split needed for the record - well all dependent on the athlete right but in the case of J.I that's a stupid statement (but this guy you are responding to unfortunately thinks he knows more than he does). I don't know how you draw that conclusion though with a guy whose strength is churning out even paced laps and runs best when he does exactly that. No Jakob can't go through in over 2.46 and rip off a sub 40 last 300m, but he might be able to go through in 2.45.2 and run a 40.6. Not sure he necessarily needs to split under 1.50 at 800m for that to happen. I could definitely see a scenario where he could run 55.2, 55.1, 54.9 - like that doesn't guarantee a record at all - but he's physically able to do this and it's his best shot.