...even though they fit a 50 amp receptacle the wire inside them is not rated for 50 amps.
I don't know about that. The last time I looked at a NEMA 14-50 range cord, the hots were 6 awg, the neutral was 8 awg, and the ground was 10 awg. Table 400.5 states the ampacity of 6 awg(whatever type of cord other than TPT, TST, HPD, HPN, etc, etc.) as 45A for Column A (more than 2 conductors) and 55A for Column B (2 conductors). Since a floor sander uses only 2 conductors, it doesn't matter that the cord came from the factory with 3. I wouldn't nitpick. I'd just give that floor sander guy a medal for being the first floor sander I've ever known to actually bother to try
to do things right, instead of reflexively
reaching for the %^$#&^% alligator clips when there is already a range or dryer outlet available!
Besides, the nameplate current of a 5hp motor is 28A according to Table 430.148. That means there is nothing wrong with using it on a 30A circuit, as long is it is not a continuous load (and isn't the continuous load rule waived under Article 527 (temporary wiring)?
An example of where the standard range cord would be inappropriate at 50A would be for an RV, since the full neutral ampacity is needed. (Never mind that a 6' long range cord would not be of much benefit as an RV extension cord!)