Just to add my two cents: I play enough games where ELO is either very fluid ... or sub-accounts/smurf accounts abound.
Starting with the former, for example, luck-based games. Sure, actual skill and reading the flow of the game (and for some, luck in itself) can be considered factors that should affect defined skill to improve rating. But sometimes, no matter how good you can guess with your luck, it doesn't turn out that way. It's rare to see master-rated players in such games, let's say King of Tokyo, for example, or even better, Can't Stop, a game basically 1000% luck-based. As such, it's obviously very possible to lose enough to drop back to 100 pts, the threshold between beginner and apprentice, when you can be pretty strong at the game.
Luck aside, sub-accounts and smurfs are VERY much a thing from what I can tell. If nothing else, Hanabi should say it all

Personally, I've met plenty of people who are fresh out of the oven with 100 pts and literally already know avalanche/black (the standard "upper level" variants), and plenty of those are smurf or new accounts. Some of them are even obviously antagonistic (blocked, RT'd, and reported, ofc) and claim they were over 1000 pts before blah blah. Whatever the case, I know there's more than plenty of low level folks who actually know their stuff, and operate as such. Of course, there's always actual beginners mixed in, who haven't practiced via training mode, and it's not like I can personally tell who is which. I used to make Hanabi rooms open to everyone, even "Beginners", until it became a little too tiring to ask if they were familiar (to Hanabi and/or variants) or not and wanted to learn. Now, I deal with things by making rooms open to Apprentice or higher, assuming that everyone who accepts to start the game knows the basics and probably the variants. I get a lot less people who *
don't* know even the basics and a *
whole lot more* who I give hints to with the assumption they know things like "this 5 on the left right at the start means play black 5". Probably less than 1 in 15, or 20 who have questions. Usually, if there is doubt they don't understand, it happens within the first several turns when someone misfires black 1 or a colored 5 :') and we clarify it then and there (if they're open to responding....some aren't, as others have said).
So in conclusion, what I have to say is, don't operate with the thought that "beginners" are anything concrete. They could be whatever for all you know. However, if a player has more than a few games and they're still at "apprentice", then maybe you can pull your punches then.
TL;DR Don't look at the rate, look at their game count vs their rate

And for "beginners", especially don't assume things, because everyone was once a "beginner"...maybe you yourself practiced in training mode with friends for a long time before going out into the rated world.