The disclaimer is there on the beta firmware pages on archive.org from a year ago, meaning that it was a general policy for beta firmware that had been in place for some time rather than something added specifically for this particular beta.
Why did this clause exist to begin with? Why should installing firmware that Asus itself publicly released void your warranty? I doubt that clause can even be legally enforced.
I can't say for sure but I'd guess it was put in place by the legal team as a form of ass covering, since betas aren't always tested as extensively and there's rarely a good reason for normal users to want to install them.
It's impossible to really know because we don't have mind reading capabilities, but he believes that their intention was never to void warranties for this.
I haven’t watched the content OP is mentioning, but Linus has a lot of industry connections so it seems very believable he’d be able to get in touch with someone who knows more about the situation.
