Music rule

So this is sheet music for trombone, you can tell by the slide position suggestions above certain notes. I assume they expect the player to have an F attachment, because 0 position is F, however the note is G, so they would need to press down the trigger to hit it (I think , it's been a moment since I've played a horn with an F attachment). Some of those symbols I'm not as familiar with.
Maybe the joke is they play the trombone, which intrinsically is a joke of an instrument.
- It looks similar to a fragment of a minuet from Brahms. I recognized it from an elementary study book of mine (Suzuki vol. 2 for violin). It gave me nostalgia and I had to find it...
  So, because it's in an elementary study book (assuming it is easy for trombone as well), maybe the joke is that when practicing alone you go for easy things that you like instead of what you should be practicing.
  Whole piece for reference:
  
  That is it, but my joke is only about the face in the middle of the picture.
  
  You know, violin makes more sense- half of those slurs make 0 sense on trombone.
- I have a King trigger trombone in storage that I haven't played in a couple of decades and change. I kind of want to go get it and give this a shot to see if I get the joke. But that may be more effort than I'm willing to put in.
- I've been playing trombone for 23 years. There is no 0th position.
- It looks like violin fingerings to me.
- Is it perhaps the "womp womp" sound on the trombone?
  https://www.myinstants.com/en/instant/sad-trombone/