It’s not. Totally not. I can’t count how many new people are there in the raids I was, some of them just recently hit level 80 and instantly joined a raid, because they wanted to learn something and have a challange and we wanted to teach them what to do. It worked, we helped them learn the mechanics and they helped us filling spots in the raid that were empty (basically making the raid half experienced players and half non experienced players).
You don’t need months to learn a raid mechanic, you might need some tries to get a raid working with so many new and random players that you haven’t met yet but the mechanics are explained in max. 10 minutes. If you don’t kill the boss first try then try it again, and again and again and again. There are enough information on the internet, videos, threads, guides, everything. It’s not that you have to learn to do these raids from scratch, like trying out something completely new. The knowledge is there, it’s easy accessible, just ask in the global chat if someone can explain a raid for you, there will be plenty of helpful people who will explain everything, even with details you probably never need to worry about.
As I already said many times now: I have seen lots of new players get the rewards already because they invested some time into the game, they made friends ingame and helped built up a raid community and got their rewards.