I'm not even sure what the OP means. Perhaps badly worded, but "power down upon takeoff" seems to mean can have on during taxi, but need to turn off "upon takeoff"...meaning the end of the roll up the runway?

I'm assuming the actual wording was something different, and rather than having just turn to airplane mode on door close, FA wanted all phones off.

Not sure what carrier this is, but I fly to YYZ multiple times a year on UA, often enough on RJs (Over the years been on multiple carriers including GoJet, Skywest, Shuttle America and Mesa), and have never heard of this before. It is certainly possible that, based on the fact each individual carrier files their own set of rules, that one has chosen to or changed to have all phones off on take off for international travel, but would find that very odd. While I haven't personally heard of this, rule allowing phones to be on also has the caveat that crews, presumably captain, can require all electronics to be off in certain cases (sever weather?), though doesn't sound like that's the explanation the FA gave. Whether the FA was right or wrong on this (I'll go with latter), passengers are required to follow crew member instructions - these days, IMO, much easier and better to oblige and be inconvenienced for a few minutes and report later than to argue.

This is my experience as well...earbuds connected to the IFE is ok, but not otherwise. My guess is its ok for IFE since it stops the entertainment and forces people to listen to safety announcements vs. the ability to noise-cancel the important safety info. out with headphones connected to your own device which could be at full blast.