Unfortunately, this is currently only true for small amounts. Theoretically, you can use any amount, but I'm sure that most casinos will ask for KYC (at least if you win and try to make a withdrawal) so you should always be prepared for wasting time on this procedure plus being able to pass KYC - if you are under 18 or gambling is prohibited in your country, then you need to have a fake identity in advance.
Yes, if you are a high roller, you should expect that the casino will ask KYC from you especially those with gambling license. So it is not true that you can deposit as much money as you want without KYC. And you need to be careful with casinos that are not active in the forum, as your funds may be at jeopardy if you send big amount. You have no way of reaching out to them if they suddenly freeze your account and decided not to answer your ticket.
In all actuality, most high rollers choose website that doesn't require KYC even when the platform is legitimate or licensed. Also, there are some deals whereas the high roller either has been some kind of deal with the website to avoid KYC process or either used fake ID or other people identity for the KYC process.
Also, it is somehow false to say that if a gambling does not have or even is not active on there forum means that your funds may be in Jeopardy as there are some gambling casino out there that has superb customer service even though not being active on the forum. There are also gambling casino that are active on the forum but has alot of report on the website such as not being able to withdraw funds or even scamming their users.