Many spa instructions specifically say to use #6cu, even though this is overkill for 50 amps. #8 al is undersized for 50 amps but this should have no bearing on the spa. The manufacturer has no say in what you feed a panel with and is looking for a scapegoat. The fact that they think something is not to code is no reason to void a warranty due to a leak. If it were an electrical problem I can see them causing a stink.
I can't refrence the exact article but it's right in 680. Within the structure conventional wiring methods can be used. Romex and aluminum wire included. Once you exit the structure the pool/spa codes become stringent.
We run 8/3 for spa feeds unless #6 is specified. Then 4-#8's (or #6's) in sealtite to the spa itself from the disconnect.
"Anyone who has never made a mistake has never tried anything new." -Albert Einstein