True. I assumed it was a simple adaptor for TB3 to 40Gbps Ethernet or atleast QSFP+ Since TB also advertises 40 Gbps, but looks like expensive niche. At $1500, you might as well spend time to figure out how to get Linux to work better with Thunderbolt card.
For other devices in the network that do not impose Thunderbolt though, Melanox 40G NICs are surprisingly cheap.
People talking about cheap 10G/40G are getting it used on eBay. They have a bad habit of comparing used prices against new prices without disclosing it. For example, if you're talking about new equipment, 40G is obsolete and you should buy 25G/50G instead. But used 40G is super-cheap because it's obsolete.
For other devices in the network that do not impose Thunderbolt though, Melanox 40G NICs are surprisingly cheap.