Yes, it’s pretty vague. Often seems to read rather like the English word “things” (another vague word whose scope depends a lot on context, e.g. “tidy up your things” has a rather different meaning to “all things are impermanent” ).
I guess it must be some broadening of scope. Something like: “all conditioned things are impermanent and unsatisfactory” being followed by “all things are anatta”. I suppose it is well-established already elsewhere that conditioned things, e.g. the khandas, are anatta. I guess the point is that anatta’s scope is broader than conditioned things. I suppose one can lump some kinds of universal invariances (dhamma principles, four noble truths etc.) in there. It’s not absolutely certain that nibbana has to be in there (given dhamma is such a vague term). Universal overself ideas have argued anatta is restricted to the khandas. But seems more plausible to me that nibbana is included here as a dhamma. Being designated a dhamma is so vague as to IMO imply very little about nibbana anyway (for those who might worry about that).
Logically, if nibbana is included under the dhamma category here, then all I think that this necessarily implies about nibbana is that it is not anicca and not dukkha and not conditioned (since it then needs a separate category). Surely, not controversial. However, not anicca and not dukkha are not necessarily the same as nibbana actually being ontologically nicca and sukha. Of course, it doesn’t rule those out either.