You just don't want to do it regularly. It was an issue for a brief time when SSDs were new, but modern operating systems are smart enough to exclude SSDs from scheduled defrags.

Defragging an SSD on a modern OS just runs a TRIM command. So probably when you wanted to shrink the windows partition, there was still a bunch of garbage data on the SSD that was "marked for deletion" but didn't fully go through the entire delete cycle of the SSD.

So "windows being funky" was just it making you do a "defragmentation" for the purpose of trimming to prepare to partition it. But I don't really see why they don't just do a TRIM inside the partition process, instead of making you do it manually through defrag

That kinda makes sense. Putting all the partition sectors together would probably make it easier to resize. But as standard maintenance it's like changing the oil on an electric car.