Girl Power Station stands out because it's one of the few entirely combat-focused levels. Talking from experience with my nephew, who just wanted to shoot things and would constantly hand me the controller for prolonged stretches because there was platforming in the way which he had no interest in. I wouldn't recommend Splatoon 3's single-player campaign to someone looking for a platformer. But I also wouldn't recommend it to someone looking for a shooter campaign without asking some important questions first. Doom, though? No second thought about telling a fan of shooters to go play it. It's a question of degrees and if you acknowledge that, of course, there is a maked difference between how platforming and combat are designed, and how prominent each is, between the two games, I don't understand how you also hold that the arguments in favor of Splatoon as a platformer apply in the same way, with the same strength, to Doom. Because it does matter whether the mix is 50-50 or 70-30.
I agree that Splatoon 3 is harder to recommend to a player with no prior experience with the franchise than Doom Eternal is, but that has nothing to do with the amount or nature of the platforming within it. It's a very idiosyncratic take on the TPS/online shooter genre, and it has yet to spawn a legion of imitators or become ubiquitous itself outside of Japan. Doom Eternal, on the other hand, is the result of decades of franchise iteration, and also partially reflects the evolution of a genre it more or less invented. It's a known commodity, outside of the increased emphasis in Eternal on juggling a variety of interlocking systems.
But platforming mechanics within a shooter is hardly unique to Splatoon, or Doom, or anything else. You can find it all over the genre - including, indeed, in R&C. The amount/distribution of platforming may be relevant to this discussion, but I don't see how it helps your argument here, given how much pure platforming there is in Doom Eternal. I don't really care whether people label any of these games as platformers. They are what they are, regardless of the words we use to describe them in shorthand. But I do think it's interesting that Ratchet & Clank: Rift Apart is regularly cited as a 3D platformer, whereas Doom Eternal essentially never is, and I suspect this is largely due to set dressing. R&C stars an anthropomorphic animal and hails from a period when mascot platformers were a thriving genre. It looks the part, and so it gets the label despite not really
playing the part.
(Although I honestly don't think I've ever seen Splatoon referred to as a platformer before, so I find its inclusion in this thread to be a little odd.)
The mechanics argument ignores that mechanics can lend themselves to different genres. Like Mario Kart's mechanics are repurposed for an arena style shooter in classic battle mode. Or the earlier example of how you could build a series of levels based on Rocket Jump challenges, and then that would be a platformer, not a shooter. Splatoon's movement and ink and shooting mechanics, and their interaction, are exceptionally adaptable, and you can build a shooter with them, a platformer, a stealth game, and other things. And if you know Girl Power Station, you also know that there are levels with single-player exclusive platformer gimmicks that don't show up in any online stage.
I can't think of a single platformer (or what I would consider a platformer, anyway) that would work at all as a competitive, combat-focused game the way Splatoon does. There's combat in Mario, Astro Bot, Crash, etc., but it's largely perfunctory. Outside of boss fights, enemies are obstacles that you can optionally engage in very simplistic combat, or avoid entirely, or in some cases even use as platforms. But the combat itself, when engaged, is not interesting, because it isn't the point.
Splatoon was clearly designed first and foremost around Turf War. This is plainly evident in the lack of cohesion within the single-player modes, something you yourself touch on above. I don't know if I would categorize the campaign as a TPS either, as it seems to shift genres from moment to moment. I love Splatoon, but I think it's struggled to establish any real identity for its single-player outside of it being an introduction to the basic mechanics in the game, and I would not recommend the games to anyone if they don't have an interest in online multiplayer. The DLC is a little more interesting, but mostly just for the increased challenge. The core mechanics may be malleable enough to support a pure platformer, but they're rarely employed for that purpose.