Reaper has it's own specific plugin format and language, JSFX (no relation to javascript), so that's probably why they specify it uses the VST format. But also relies on Reaper specific APIs making it only work in Reaper.
JSFX are distributed as text files, not binaries, and they can even be edited on the fly while the DAW is running. I don't think they can be called VSTs, but they sure are extremely powerful (I have written a couple myself).
JSFX can also be run in most DAWs using ReaJS, a JSFX VST host written by Cockos (not updated in a while, so some recent features of the language won't work), or another similar thing called YSFX.
Playtime probably uses Reaper-specific API calls and UI manipulation, but why the author chose to write a Reaper-specific VST instead of a plugin, IDK.
They aren't VSTs but that's my point. A "Reaper plugin" to the community usually means a JSFX plugin.
Playtime is a VST plugin, not a JSFX plugin, but it also relies on Reaper specific API calls making it incompatible with other DAWs that support VST.
It matters because you'll find it in the VST section of your Reaper plugin list and I believe Steinberg also requires you to use the VST branding on any plugins using the VST SDK
VST isn't an open plugin format and it doesn't guarantee compatibility across all hosts. It's a source-available format owned by Steinberg who grants financial free use of it (but not free use in the FOSS sense)
Reaper has some VST(TM) extensions that only it supports. So if you create a VST(TM) plugin that uses one of them, it will not run (correctly) in other hosts.
