Saying React can't do it, to paraphrase you, because "technically it's harder than with Vue" is like saying nobody can drive a stick shift because you have to shift gears manually.
Yes, you have to do it manually, no it doesn't make the car undrivable.
That's what I call FUD.
The car gear analogy is again downplaying magnitude. The examples I gave are not a comparison between auto vs manual, it's more like auto vs getting a different license type to drive a 18 wheeler and having to learn how to turn, back out, park, go under bridges and where you're allowed to drive and not all over again. Yes, it's technically doable, no it's anywhere near similar amounts of effort as just learning stick.
To clarify, again, just because it feels like react vs vue is akin to manual vs auto when you're in a codebase that is amenable to being refactored into react, it doesn't change the fact that there are types of codebases (non-SPAs without HTTP API layers) where migrating to react is a significantly larger investment than vue. That was what the OP was saying.
React wasn't even mentioned, don't know why so defensive, who cares
I'm seeing a bunch of commenters getting defensive and saying "well technically" while conveniently ignoring the difference in effort required to do islands in each framework. You can't say dropping JSX is at all comparable to plopping in v-clicks on existing HTML.
You can certainly adopt react "incrementally", in the sense that you can implement an entire new self-contained feature and plop it into an existing app if it already uses REST APIs, but react is decidedly not as trivial to sprinkle into a non-SPA where data comes mixed with HTML rendered on the server.