Maybe I am missing something, but the one trailer that had Matthew slurring didn't alarm me. They were talking about getting emotional, and I thought he was playing it for laughs a little. I didn't see it as his trying and failing to say something. They had those kinds of moments on Friends a lot where someone was so upset that they no longer made sense.
Does he sound perfect in the cameo video above? No, but nothing that alarms me. Are there other clips out there that raised alarms, or are people deciding he has relapsed based on that one moment and his usual disheveled appearance?