Can POS devices really generate random numbers?
Modern POS devices are designed to meet the strict security requirements of payments industry (search for PCI-DSS standards). They are typically based on a SoC designed specifically for secure payment applications. An example of such SoC:
https://www.broadcom.com/products/embedded-and-networking-processors/secure/bcm5892/#overview

From the datasheet:
"The random number generator (RNG) is a true random source. It utilizes free-running oscillators to capture thermal noise as the source of randomness. It has been used in numerous devices (such as BCM5820, BCM5821, BCM5841, or BCM5890) and been evaluated as a valid source for seeding FIPS approved pseudo-random number generators as the initial secret value"

If the POS in question runs Linux - it's fairly new, but if in doubt you can always query its vendor directly.