The sales info about these cars is not very clear at all but it does clearly say that the engine has been replaced.
I was under the impression that that Japanese market were more interested in RHD fairlady's than US market 240z's.
The engine in the car was never changed to an RB26. It's a bad translation on the English pages of the auction blurb. The Japanese pages make the situation clearer, but more importantly the multiple feet on the ground at the event reported fully on specs and condition. There were some changes to the cars (including fitting one with a 5-speed transmission) but the engines were stock, with all emissions control stuff present.
99/100 re-imported North American market Zs sell for way less in Japan than their equivalent date/condition Japanese market counterparts. They are just not that popular and are seen as something slightly 'other'. Japan doesn't have a problem with LHD cars in general, and there are millions of parallel import Euro LHD cars in Japan because they were cheaper than the proper RHD Japanese market versions from the official MB, Audi, VW, Porsche, Renault, Peugeot etc etc Japan dealerships. It just doesn't work with Japanese cars, and especially old ones.
These two cars were parts of a typical auction 'perfect storm', hyped up in a Tokyo Auto Salon-affiliated event (so lots of 'car guys' in town), a well-known previous owner and being 'Vintage Z Program' cars presented in faux 'Barn Find' condition. It probably matters not to the buyer(s), but they could probably have bought better cars for less money elsewhere.