Maybe they have "roger" confused with "copy", which is widely used but incorrect.

The thing that you never say in aviation radio communications is "10-4". That's for CB. Roger, wilco, "no joy", and "tally ho" are used all the time. No joy means "I don't see the traffic you just told me about", and "tally ho" means "I see the traffic." This guy has a bunch of videos that are pretty good for radio comms: https://www.youtube.com/channel/UCxGjm4p2paQEhuyuVp_woDA

Thanks for the video. I did my PPL in the UK so coming to America was a bit of a radio culture shock. I think "looking for traffic" and "traffic in sight" are the ICAO phraseology, and will work everywhere. Flying to France, you might confuse a French controller by saying "tally ho".

Having thought about it some more, the statement about "roger" not being allowed was probably missing some context. He probably meant it in the context of an ATC instruction that requires reading back.

What's incorrect about "copy"?

It isn't ICAO phraseology, it's a military term.

Gotcha, so it's correct in terms of its meaning, but incorrect in that you're not supposed to use it in this context.

I think the author got a bit confused, as the next paragraph (quoting the VATSIM ATC) clarifies this statement... in the example given the ATC instruction requires readback. "Roger" is not what the controller is looking for in that case.