can Slenderman speak in stag form?

[Axelmania]

China 1 (Yama Kings) pg 94 mentions that the Long-Armed Giant can speak while in tree form. It doesn't say this while they are stag form, but it doesn't say they're mute either, any opinions either way?

That seems like the most convenient form since it's merely 7ft tall 12 ft long compared to 30ft tall in humanoid form. People would probably be less terrified of a helpful stag than a pale giant with arms down to its feet.

If they can't speak, does that mean they can't cast spells in stag form, or would they have to rely on the silent casting rules in Underseas to take more time for low/mid level ones? (High level ones don't seem to take more)