To be honest, I am surprised that something like this is not immediately visible.
It turns out that when meson doesn’t find ffmpeg dependencies installed and findable by cmake/pkgconfig (libavfiler etc.), it will clone it and I think it will build a license-restricted version of it enabling only a single element which is the avdeinterlace.
Installing ffmpeg from apt via apt install ffmpeg libavfilter-dev then building gstreamer with the mentioned commands will enable all the plugins/elements from libgstlibav.so
Thanks!