It’s not libel. Defamation requires a false statement of fact. Claiming a website is “unsafe” is an opinion.

(IAAL, but this is not legal advice. Consult a licensed attorney for legal advice.)

because google safe browsing is only supposed to display a "not safe to browse" warning when using chrome browsers (and maybe some other browsers) wich you can (theoretically) dismiss(1)

it's not meant to have any other consequences

so basically what happens is that because of hearsay of google thinking you site is not bad Radix does what normally should involve a judge order (taking down the whole domain)

(1): Yes that still would cause damages on any site with customers, but like way less and way more fixable then what happened here.

This is libel, indeed.