It is true and for a short moment in time it was possible to upgrade LH P or Z fare class to F with just one GPU (pre-covid). I recall that this required quite a bit of convincing back then because the website did not state that. I was able to use that two times. But now the minimum required fare class is D, C or J.
That was a short term glitch when LH switched on their side to allow P and Z, but this was never meant to work with GPUs. D, C and J has been the rule for quite some time - and they clear with 90+%.