It’s not known to be a backdoor, but it’s a juicy attack surface that customers are largely ignorant of and provides little consumer benefit. If I were an NSA employee and my boss handed me a blank check to develop a preboot exploit for Intel PCs, I’d start with IME.
$ :|wc -c 0 $ touch /tmp/f; :>>/tmp/f; wc -c /tmp/f 0 /tmp/f