Based on labs I suppose it does lean more liver disease, so it's absolutely worth trying the Denamarin and ursodiol.
But it could also be cancer, kidneys, or heart (or something else, not sure, not a vet either). Kidneys are less likely since they look pretty good, and labs don't look like cancer but it doesn't always show up in lab work. You said they didn't hear a murmur, so odds are heart is ok but not guaranteed.
Did they have an estimate of how much fluid was in there? Is it impacting other organs? Does it seem to be increasing over time? Also wondering why they didn't drain any fluid (there's pros and cons), but as you said they kind of wrote him off.