Not every yatzer hora is assur. The Tanya stating that the neshoma is clothed in tumah when learning secular studies that aren’t lisem shomayim, also doesn’t mean that it’s assur.
Bein hasdorim means not when learning. The question wasn’t about making a seder in those studies, but rather to do it when they’re not busy. If the questioner had phrased it “every day at 1pm” it would be a kevius and would be assur.
It’s not “my pshat” – if you saw the word in any other context, like eating outside a Sukkah, eating before bedikas chometz…we wouldn’t be having this conversation.
What do you call a desire to do something not for Hashem? Lf course it’s a yatzer hora. Rav Moshe allows a lot of things which aren’t ideal or the best thing to do. He allows a man and woman to live in the same house in separate rooms to “test” themselves to see if they should get married. Does he recommend it? He knows his audience. If he didn’t allow it they’d probably do it be’issur. American jews had American ta’avos, biases, and challenges. It took someone with tremendous “playtzes” to know where and when to draw the line.