AMD (and Intel and everyone else) processors already have an HSM inside for confidential computing so use that? I would hope the HSM isn't as badly implemented as this update mechanism, but then again ...
I'm happy enough with TLS introduced: knowing the server I'm reaching for updates is actually 'amd.com'. Signatures would be nice, sure, but I wouldn't consider them nearly as critical or useful until now. Before we get too caught up in signatures, however, I'd like to see their new/improved updater actually take precedence.
As things stand, I'm not sure key rotation would go well... the updater doesn't mind itself, apparently.