Well, that’s of course at least a lot of FUD, if not BS.
Purism has proven that they keep updating their devices. And even by now, almost everything they developed is mainlined (Linux kernel) and up-streamed (Gnome).
I mean… if somebody basically titles a post “Don’t support a company that tries to free up as much as possible” because (supposedly) an existing alternative might offer you more, but not because the vendor stands for freedom, but because you can flash an alternate OS on it… what argument is that?
Purism (in contrast to Google) does stand for freedom and also allows you to flash on it whatever you want.
By the end of the year, you can probably already choose between PureOS, UBports, PostmarketOS, and maybe even LineageOS or Replicant (Note that the Replicant page says that they don’t provive ready-made images to flash, because for all known devices, binary blobs would have to be included - the Librem 5 could be the very first device for which this restriction does not apply, and thus an image can be made available).