Actually, inspection of the code shows that this error message occurs all the same for any symbolic entries like here:
> A:=Matrix( [[2,a],[b,1]]):
sys := State Space (continuous)
Error, (in DynamicSystems:-Grammians) symbolic value(s) in model: a, b
Meaning that whether these symbolic entries are function calls of t, or something else, makes no difference.
Yes, there is nothing in the help page that says that symbolic entries, function calls included, are not supported. Actually, it is frequently the case that help pages do not specify or are ambiguous about the admisible input types.