We all know it's not the 'leaders' that negotiate - they turn up to glad hand/take the camera opportunity etc.
Negotiators succeed only because each party recognises their opposites have the authority to make a deal and the ability to sell it to their leaders. Mo's wig flinging would have been irrelevant if she wasn't trusted to get the deal through.
The thing is, the nature of the pfeffel/cummings government is well understood - nothing that frost et al propose can be assumed to survive a day's bumbling with the leadership - so Barnier has no incentive at all to move beyond the centre of his mandate.
Barnier is just going through the motions.
(cummings wearing a suit.... In my experience, when one of the 'mandatory dress down' crowd cleaned up it was because they'd got another job, or were going to court, - we can hope.)