I remember when postmarketOS has been created, but that was already pretty late in my book :) You completely forget about earlier porting pioneers, such as SHR that worked on devices like HTC Dream, Nexus S or Palm Pre (and GTA01/02/04, obviously); or even projects that predate my personal involvement, like OpenEZX. We've seen it happening multiple times by now, so there's a lot of insight to take from that already.
> had other running phones way before like the Nexus 5
It is pretty clear that you haven't actually used pmOS back then (2017-18), as none of the supported devices could even do phone calls there, and that was far from the only missing basic feature. Which DE would you even run on it for daily usage, Hildon? ;) It sure was an amazing project with a lot of promise already, but absolutely nowhere near being actually usable (aside of some PDA-like use cases). Back then the only linuxy OS that you could actually use on some real-world devices (not counting proprietary ones like Sailfish or already obsolete ones like SHR) was Ubuntu Touch, which was pretty far from the regular desktop architecture (still is and doesn't intend to change that) that's pitched by distros like pmOS, PureOS or Mobian. Things started to move with the advent of Librem 5 (2018) and PinePhone (2020), thanks to Purism investing resources into creating phosh and maintaining supporting infrastructure in GNOME, and Plasma Mobile experiencing an influx of new developers and moving away from Hybris when PinePhone started to become widely available. That work is now directly consumed by ports of pmOS to other devices as well.
> what evidence is there they didn’t just drain resources from other projects
So, considering all I said above - which other projects? Distros like postmarketOS have been fueled by L5 and PP, not drained. Work on mobile GNOME has been paid for with Librem 5's price. Plasma Mobile was a tech demo until more people started to hack it on the PinePhone. This should be evident to anyone who actually watched that space unfold across the last 15 years. These days we can even start hoping for working phone calls on ported devices like OnePlus 6T, but where do you think the ModemManager code to handle phone calls actually came from? ;]