I guess I fail to understand how you think that he's hiding anything. He's literally telling everyone that he works with ZMF and asking our thoughts on how to handle it. The fact that we're even discussing it at all means he's not hiding anything and therefore can't be accused of lying.
Right now WE all see and hear him asking, but if he "ghosts" ZMF, especially after having previously reviewed them,
in a year+ with nary a mention of ZMF, if he then reviews a few headphones, doesn't compare them to ZMF, and someone easily figures out he's a ZMF distributor, they may rightly wonder why he never mentions a product he sells. Further they conclude bad intentions, that he's HIDING that fact. Then they wonder: if he's hiding his relationship here, what else is he hiding?
Further, even by disclosing he's NOT comparing or reviewing ZMF he risks an obvious question: "why not? Because they suck? Because he doesn't believe in something he sells? What's ethical about hiding it? He must have some other agenda ...." confusion, uncertainty, doubt.
So by "ghosting" he creates a situation where we never know what his interests are because he specifically has a policy of omitting them: "Gee, did I miss the video or post where he told me about it?"
Again, this is why people use disclosure: it ensures there's never ambiguity of interests. Change of behavior always risks implying otherwise.
Further, the only reason we're discussing this is because he's asking if he should use an ethical standard OTHER THAN the normal business ethical standard: disclosure.
Simple: change nothing, review what you want, compare to whatever you want, BUT DISCLOSE.