> The author claims they can be had for only 20 USD online. That seems too good to be true!

In my experience, the cheap eBay MLX cards are DellEMC/HPE/etc OEM cards. However I also encountered zero problems cross-flashing those cards back to generic Mellanox firmware. I'm running several of those cross-flashed CX-4 Lx cards going on six or seven years now and they've been totally bulletproof.