yeah the main reason that they are comparing wii U to ps5 is becasue... japan was by far the wii U's most healthy market, to the point that it took the ps4 untill basically the announcement of the switch for it to catch up, as you can see...
The main reason why people compare the PS5 to the Wii U is that the Wii U was a failure. There was nothing healthy about the Wii U in Japan as you should be able to tell from lifetime sales of ~3.3m.
The PS5 is tracking behind the PS4 which itself tracked behind the Wii U for a long time, not just in the above graph, but also launch-aligned. Anytime a new console is compared to a failure, it's about making the point how bad the current sales are.
What the PS5 is doing right now is make up for the initial shortages. You can tell from @TruckOSaurus comparison tables that the PS4 didn't taper off to its eventual weekly baseline until it had hit ~500k LTD, so the PS5's current 400k+ LTD is still below the threshold where the entirety of the initial demand should be fulfilled. Taking this a bit further though, there are PS5 shortages in the rest of Asia too, so a good chunk of the PS5s sold in Japan are bound to be leaving the country immediately, hence why physical PS5 software isn't charting in Japan's top 30. In that sense, the current PS5 hardware sales don't paint an accurate picture of the health of the ecosystem in Japan. Decent hardware sales are better than bad hardware and software sales, but what we are seeing right now still can't be encouraging for Japanese third parties who want to develop PS5 games.