While you are generally correct, stating things that are mostly true, those things are not the issue here.
Nintendo creates demos, ties them to online play & releases them before the game comes out. Then they pull the demo a few days later. The game goes on sale. Months pass. Then they release the same demo again, this time without the online enforced time bomb. This is what they did with ARMS. There's a pattern of Nintendo doing this kind of thing, being unimaginably stingy with demos. It goes back quite some time.
Demos still get created. Builds for media events and public hands on and whatnot. This demo is that, a slice of the game Nintendo used at various events pre-release to market the game.
Back to your point: In the not-too-distant past (last generation) pre-release demos were rarely released to the general public on account of lots of data showing that the availability of a demo actually hurts sales. That calculus has changed this generation, with the advent of the always connected game as a service. Now, the vast majority of AAA titles of note offer a pre-release demo in the form of a fake* beta, offered briefly before release for a limited time span. Once the game has back, that slice of content tends to return as 'always available' demo.
Nintendo is still stuck in that old mode of creating titles that sell once, get played, get put on a shelf. They rely on the long tail rather than the smarter approach of creating a title that evolves over time, consistently being played (and generating revenue) from/by a growing audience. So they operate how others used to operate - creating demos, allowing them to be played at events but rarely releasing them to the broader public. Even judged by that old standard, Nintendo is way stinger than the rest of the industry.
*Offered far too late to have any meaningful impact on game design decisions/non-blocking bugs/etc. Used as a marketing tool and a way to evaluate the backend services, to ensure that they can cope with the demands a real player base places on them.