So I was discussing this issue with a friend, and he has the theory that the cause of other files appearing in the Nintendo Switch's files could be due to the way it implements functionality similar to Linux's ftruncate. ftruncate is a function that truncates a file to a specified length, and the Nintendo Switch could be using this to extend the length of these files. Now, since FAT32 (not sure about exFAT; perhaps someone try this with an SDXC card that was once in a smartphone?) does not support sparse files, the OS has to pre-allocate the space to the files. Normally, the OS should initialize this new space to NULs (0's), but in the interest of speed, perhaps Nintendo omitted that favour for performance in their implementation of this functionality. From what I've seen here, everyone who's tested has done a quick format, which does not wipe the old files' contents. Because of this, there is a high chance that the previous files' data still remain in this new segment of the file.
TL;DR: this data could be left over from a previous format as a result of an implementation detail, and any foreign, recognizable data is probably a red herring.