So when you select coaches, it shows the skills which they utilize for their position group in red. Defensive Coordinators have "Punt Strength" included. Is this intended or accidental?

Without looking, possibly for fake punts?

From the guide info, punt strength deals strictly with how far a punter is capable of kicking the ball.

I'm guessing that STs come under their D duties as there isn't an STs coach

The kickers coach covers both punters and kickers, so is more or less the de facto STs coach, as your other ST players all fall under the development purview of their respective primary positions. If the D Coord is expected to be the STs coach, he needs the rest of the ST skills listed, rather than just the one all on its lonesome.

Definitely needs confirming, it does seem odd

You're right, that's not supposed to be red on that card. I'm surprised it has taken this long to get noticed! I have fixed it. Thanks for reporting it!