It was just a matter of time, considering Ryzen has become popular enough to warrant serious vulnerability research and in this case, it was the research project of a graduate/postgraduate hoping for a PhD.
They were also responsible enough to at least give AMD advance notice in August 2019 (for any title-only readers), so it'll be interesting to see if AMD already addressed this quietly via past BIOS updates (IE: Doing their job without needing to be prodded like Intel) and improved hardware security for Zen 3 and 4 (and even maybe newer production stock, like the AF 1200/1600 and newer batches of 2000 and 3000 series).
That said, it's nice that Intel is indirectly providing bug bounties on AMD's behalf. Sure, AMD takes a minor hit from sensationalist titles once a vulnerability is found, but they also don't have to pay for bug bounties themselves. And like this research paper, digging into Ryzen for that PhD is fresh territory, as opposed to trying a paper on a new Intel vulnerability.
Considering AMD has 2 active teams that compete with each other on Ryzen development, the sooner any vulnerabilities are discovered, the faster AMD can respond and amend in-progress development.