"The original Last Ninja sold a huge amount of units - I think it reached 4 million on the C64. Ninja 2 came out right at the point when the C64 was at its most successful in terms of sales, and sold 5.5 million copies. If you take that into context, it’s estimated that around 20 million C64S were sold, so one in four C64 owners bought the game."
Back in 2005, he put the combined sales of the three titles at 12.5 million (he says three million for the third later on). I think it's safe to say that it has not sold any more copies since 2005. The remix of the first existed at that time, so there's no reason he wouldn't have included it then, especially if it sold so incredibly well (over 10 million if the 23 million figure is to be believed).
There are a few potential explanations, but the most likely is simply that the dude is exaggerating. I wouldn't even take the 12.5 million figure too seriously.
I do think he added all platforms together to get to 23M. While it started on the C64, it was also on a slew of other platforms:
The first one alone was already on Apple ][, Apple ][GS, Amiga, Atari ST, PC, BBC Micro and both Acorn Electron and Archimedes. The second one added NES, ZX Spectrum and Amstrad/Schneider CPC. The third one dropped the 8bit computers apart from the C64 and added th CD32 instead. Plus they were all three on the Wii Virtual Console
From what I gather, those 12.5M were only for the C64 version, and the 23M could be all versions combined, which sounds plausible to me. The C64 versions were very well loved and some of it's Killer Apps, so reaching very high sales numbers is certainly not out of the question.
"...the sales of Last Ninja 1, 2 and 3 on the C64 were over 23 million copies. That's quite impressive, because Commodore only sold just over 20 million machines in total."
He specifies the Commodore 64, and doubles down by mentioning the commodore 64 install base. Either he made a massive error in speaking, or he's lying.
And, 10 million would be a lot for those other systems especially because they were all incredibly niche products except for PC and maybe Apple. Collectively, the games would have to have averaged around 1,000,000 on each of those platforms, which would be incredibly high sales.
23 million is a pretty staggering feat. To put it in perspective, that's a little more what the original crash Bandicoot did on the PS1. So... to believe that The Last Ninja sold that much, I'm gonna need a lot more than its developer's word, especially when he is at best not great at communicating the sales and at worst telling a fish story.