The acquisition is surprising, but I doubt Nintendo has any major plans for them beyond doing what they're currently doing. If we look at Nintendo's most recent acquisitions, Next Level Games and SRD, both were bought because the owners were looking to sell, and Nintendo bought them in order to maintain the status quo. Both companies now do pretty much exactly the same thing they did before, just owned by Nintendo.
In Shiver's case, it's pretty obvious that Embracer would have been looking to sell. They've sold off a number of companies recently, and are splitting the company into three groups. Shiver doesn't really fit neatly in any of those groups, so it makes sense that they'd want to sell it off separately. From Nintendo's perspective, although they haven't worked directly with them like they did NLG or SRD, they have benefitted from their existence, as many third party studios rely on outsourcing Switch ports to companies like Shiver.
Had Nintendo not bought them, the risk would be that they would either be absorbed into a larger studio, and stop their port work, or potentially even shut down completely. This would mean fewer options for third parties to bring their games to Switch and Switch 2, and potentially fewer third party games altogether on Nintendo's platform. Nintendo could pay likely very little money (as they seem to be only a ~20 person team, and have no IP of their own) to ensure they keep doing what they're doing, and I feel like that's what they've done.
