It kind of is, though. Google doesn't randomly try to visit every URL on the internet. It follows links. Therefore, for these files to be indexed by Google, they need to be linked to from somewhere.
Can you actually explain why the phrase you cited from OP is wrong? You say that ~”files need to be linked to from somewhere” is correct. How is a file linked to from somewhere [on the internet] if it’s not being served on the internet that Google crawls (ie, HTML)? The only alternative is in… API calls? That Google probably isn’t crawling?
“Fiverr might be hosting public HTML somewhere” seems like an entirely reasonable alternative phrase to “these links must be linked from somewhere [that Google can crawl] “, at least to someone who is only superficially familiar with how search works.
The distinction you imply is obvious is not, and your point is thus rather confusing to someone who is not you.
It’s a huge mistake to assume these links have to originate from fiverr-hosted HTML, it’s far more likely Google is finding them from places like GitHub repos used by fiverr-users.
That was my first thought, but is it logical to assume that 5+ unrelated people took their finished tax return URL and linked it on a website/tweet/etc? Who would do that?
Even still, Fiverr could very well have GDPR/CCPA/etc liability as the host of these files, because they related to its services, it's not just a generic file host.
It's exactly how it works, pages don't just magically appear in Google's index.
You need links to pages either from your own website or backlinks from other websites. Alternatively if the page is in your sitemap then Google will typically pick it up or you can manually submit it for indexing. For important pages you would typically want internal links, backlinks, and have it in your sitemap.