The Stug is an assault gun not a tank destroyer. The SU 76 as vipper just said has 60 range. The m10 is not one either in this game specifically, even though its named as one . It lacks range and sight so does not fit into the TD category.
A) Pretty much all mediums/heavies have same sight, difference is in the range (50 or 55 o 60).
B) Panther is a TD. TDs in game are:
- M10 TD/Achilles
- Jackson
- Firefly
- JP4
- Panther
- Elefant/Jagdtiger
- Stug III "Initially intended as a mobile assault gun for direct-fire support for infantry, the StuG III was continually modified, and much like the later Jagdpanzer, was employed as a tank destroyer." wiki
- SU85
- SU76
You can argue about semantics all you want, but this is taken straight from the different websites that classify and give information about certain tanks. Just google it, for each one, like I have. Most of the time, tanks like Stug are called assault guns, while other sources call it tank destroyer. Assault gun is probably used because of the ammunition type it could sport.
And if you want to have "we're talking about a game, not real world". Then it still holds water.
As long as a tank in this game is good at killing other tanks, be it medium or heavy, it's a tank destroyer.
Achieving TD status you have to either:
A) Have agility to outmaneuver enemy tanks like M10 TDs and a gun to penetrate armour
B) Have a big a** gun with low AOE like the Firefly and Jackson
C) Have the agility AND the gun AND the armour to outlast and outright destroy tanks, like Panther
D) Be a chunky boy like Elefant/Jagdtiger
E) Range is optional
Of course, you can have a "in-between" class like tank hunters, but some sources use that term and other don't. E8 and Panther and SU76 would be tank hunters by that definition.