I was under the impression that Next Generation POSIX Threads were the future of threading on Linux. I know that it is a M:N implementation instead of 1:1, but I don't know much else. Did NGPT already ...

Yes, NPTL is Native Posix Thread Library. Supporting "full Linux [user-mode] processes" implies supporting the kernel pieces that support NPTL. The major pieces are the syscalls clone, set_thread_area ...