can be const

It's clearer to have non-inverted booleans, so:

const bool tweaking = (adt->flag & ADT_NLA_EDIT_ON) != 0;

This then makes the rest of the code follow naturally:

const bool soloing = ... 
const bool actionstrip_evaluated = r_action_strip->act && !soloing && !tweaking;

The code doesn't seem to be in line with the comment. From the comment, I would expect something like this:

if ((nlt->flag & NLATRACK_DISABLED) && !(contains active strip)) {
  return false;
}

As the code is now, I suspect that (adt->flag & ADT_NLA_EDIT_ON) and nlt->index != adt->act_track->index are part of the "contains active strip" aspect. If this is the case, the fact that it's split up into separate if-statements doesn't make it easier to read.

The first condition can be flipped to reduce the cognitive complexity:

if (adt->action == NULL) {
  return false;
}
if ((adt->flag & (ADT_NLA_SOLO_TRACK | ADT_NLA_EDIT_ON)) == 0 && !any_strip_evaluated) {
  /* Evaluate as if there isn't any NLA data. */
  return true;
}
return false;

You could even take it further and simplify the condition a bit as well. I'll leave it up to you which one you like best.

if (adt->action == NULL) {
  return false;
}
if (any_strip_evaluated) {
  return true;
}

/* Evaluate as if there isn't any NLA data. */
return (adt->flag & (ADT_NLA_SOLO_TRACK | ADT_NLA_EDIT_ON)) == 0;

The comment is a bit vague to me, so I don't know if I placed it over the correct part of the condition. I'll leave that up to you.