Actually if you want to be techinical about it. All passengers must be in their seats with their seatbelts fastened, seatbacks and tray tables up, and all overhead bins closed PRIOR to closing the main cabin door. The main cabin door can only be closed when the "cabin is secured" as an FAR, and all of the above must be in compliance for the cabin to be secured.

So in reality a flight attendant does have the ability to say "your seat must be up" during boarding, in securing the cabin before closing the door.

But what actually happens is a different story, and that is flight attendants may be busy during boarding counting meals, greeting pax, taking drink or meal orders (for f-class), or maybe just chatting it up with the other fa's in the galley... So it's rarely enforced and instead it is all done on pushback when the compliance check is done. If you were to have a random FAA inspector obviously you would follow the rules by the book...

In any case the OP said he was the only one asked to put their seat up, and the fellow pax also reclined weren't. So it was prob just a cranky fa, trying to take it out on the pax for having to work an all-nighter.

I like my sleep, and I like going to bed early (boring I know), as a flight attendant I hated working red-eye flights. But I was still nice to pax, considering in reality you only deal with them during boarding and deplanning, and for the most part the majority fall asleep once you turn off the lights before take-off...