Would doing the 2h30 min excercise over say, 2 or 3 days per week, amount to the same benefit?
so i'd assume that short times more often is better
I wouldn't assume this. The logical end of this is to split your exercise into many tiny bursts throughout the day. But it's likely that sustained output is an important part of exercising. The shorter your exercise, the less likely you are to get your heart into an elevated range, the less likely you are to cause an oxygen-depleted state (which encourages vascular growth), the less likely you are to use a significant portion of your free glucose (which is important for insulin sensitivity), etc.
I don't know where the threshold is, but it seems like a bad idea to assume that shorter, more often is better. It likewise seems like a bad idea to assume that longer blocks are better (or just as good). Any exercise is far better than none, but there is a lot of support for ~30 minutes ~daily being extremely valuable. Absent evidence, I'd be hesitant to assume that some other arbitrary division of exercise would yield the same results.
I don't know if this is a safe assumption either. What makes you think it would take more than a few minutes of acute exercise to reach an oxygen depleted state?
Depending on your fitness level and exercise level, it will take more or less time to put you into this state. An overweight, sedentary person sprinting will be there pretty much instantly. A marathon runner walking briskly won't get there in any amount of time. But all things being equal, more time exercising will be more likely to put you into an oxygen-depleted state (and glucose depleted state, and elevated heart rate).
https://mobile.nytimes.com/blogs/well/2016/04/27/1-minute-of...
Anything is a lot better than nothing, but there is some evidence that there are all sorts of good effects from a longer session once in a while.
The other thing about phasing in short brutal intervals for a few weeks and mostly doing Long Slow Distance ("LSD") is that it's either over quickly or it's quite pleasant. I get most of the exercise in the more painful 'middle' of this continuum from sport.
EDIT: here's the article that got me interested in LSD style training. https://www.elitefts.com/education/training/echocardiography...
Either way, I think it's easier to go for an hour 3 times a week, than doing half an hour each day. (would become a bit more pricey, and I'd lose the time of going to the pool and back each day. Not as easy to combine with other responsibilities imo)