Yes that is the correct file.
I don't know about 'date modified'. In principle I'd expect it to be 'modified' whenever you save, even if you don't do anything on the same (i.e. you load it up, save it straight away and quit as soon as you've done that)
BUT in my experience Windows isn't as accurate with modified dates and creation dates as it should be. I've had files before with a 'date modified' before 'date created', and I seem to remember being baffled by sometimes 'date modified' never being updated no matter how much I modify a file.
So I'm sure that the fact that 'date modified' isn't being updated doesn't mean anything.
The obvious way to check would be to
1) change something in the game e.g. use some ammo, get an item, make some progress
2) make a save state (with F5) as a backup in case something goes wrong
3) make a native save (with the in-game menu)
4) end emulation and close the emulator
5) open up PJ64 again, load the ROM, it automatically loads the native save. Select your file from the in-game file menu. (In fact, if your file is there at all then it means the native save is working right)
6) check to see if the change you made is there. If it isn't, you can load the save state (with F7) to restore your progress.
If your progress is there than no matter what 'date modified' says the native save system is working corectly.
And be aware that save states are usually OK but they are not a long-term reliable system of saving. They can become corrupted or cause crashes, so it's better to use native saves and shut down/re-boot the game between play sessions.
