Are the wires for my generator transfer switch undersized?
I received a good answer from the manufacturer. I wanted to post it as an additional answer. Thanks to all for the mention of UL and appliances being separate from general NEC building codes. I've learned a lot from the comments and the other answer (which I've accepted). 

UL 1008 is the standard these devices meet.

Here is the email response. They have been very professional and timely in their response and the product is well-built. I recommend them:


  Hello, thank you for the inquiry and for choosing our product!
  
  The wiring we use inside the Q-series cabinet, specified as factory
  wiring, is sized in relation to the testing for heat-rise on wires. 
  In a factory wired electrical product, the wiring within can be
  potentially rated at any size that passes the heat-rise tests set
  forth by the testing lab, in this case Underwriters Laboratories Inc. 
  6 AWG is rated for 55 amps and is required for most 50 amp
  applications when dealing with general wiring, but the wiring in the
  transfer switch does not fall under this category as it's enclosed and
  presumed to be unchanged after the factory build.  If number 12 wire
  were to be sufficient in passing all of the stringent UL testing, we
  would use that instead of number 10 wire.
  
  I understand the concern and it's true that you will see 6 AWG wire
  for most 50A applications, the number 8 and 10 wires in the Q510C are
  permitted through the listing of the product by UL as they passed all
  of the testing that is required by the national standard for transfer
  switches (manual), UL
  1008.  There are actually a lot of potential applications that could use smaller wiring than what is considered normal, but the NEC isn't
  able to differentiate every application in the codebook so they have
  very standardized wiring regulations to enhance safety and avoid
  undersized wiring.  In this scenario, we are using a smaller gauge
  wire but due to its short length and enclosure in the cabinet it can
  handle a 50A load with no problems.  Please let me know if you have
  any questions, thanks!