If you look at the list of things done before he calls it "rebuilt" you can see that it's not even up to the level of a remanufactured alternator. He just cleans and polishes the parts, skims the head, and puts it back together with new seals, apparently.
The guy has been doing the same thing for many years. He even has newer complaints on the various review sites. I saw a few for late 2021. But, he's got a good web page and knows how to promote. Doesn't care about reputation, apparently, just making the sale.
From his N42 head web page, below. The only new part is the valve seals. The heads only have a 30 day warranty. So, anyone who buys one better work fast.
"REBUILD PROCESS INCLUDES:
Complete disassembly, Inspection and Pressure test
Deck Re-Surfaced
Reconditioned Polished Rocker Arms
Cleaned and Installed OEM stock Springs, Retainers and Lash Pads
Full Set of new Valve Seals
Stock Polished Camshaft. (Performance Cam Kits available, discount offered. call for details)
All threads cleaned and repaired as needed
Full Reassembly and Valve Adjustment (Re-Adjustment Required Prior to start up)
30-day warranty
Trust your Datsun to a professional licensed Datsun Restoration business with over 15 years experience."