Marx – eating before davening, ba’krai, is in fact assur – sukkah is a real example. We know that it means a snack; now the poskim do deliberate over what a snack is, but they’d all agree that a meal is assur.
Here, we’re being shown that making a kevius of study is prohibited. A fixed study isn’t the same as “how much food”, because here the fixed study is an entirely different activity than if done casually.
What other meaningful pshat in kevius/akrai is there other than kevius = set, and akrai means “here and there”… You walk by a bookshop with an interesting book, and you read it a little. You’re in a bathroom, and you have a physics book as rav belsky had in his, or you go to a zoo or museum and learn about nature… Those are “here and there*
A bochur in yeshiva who studies science bein hasdorim will not be doing so everyday, at the same time. There are 2 periods of bein hasdorim, and I’m sure he has other things to do (like rest, if he’s actually learning, run errands, etc)
Rav Moshes whole point is that during that time, it’s not set up what you’re involved in, so you can play basketball, read the paper, read a science book, go on a walk, or do whatever it is you want to do.