Don't worry, he isn't an MEP.
He doesn't need to be, if May falls and the Tories elect a new Brexiteer PM then everything JRM has said could come to pass through the Government vetoing everything the EU tries to do... one positive that might come out of it is that it may well speed up the EU's acceptance of majority voting?
It is of course JRM/ERG's version of 'sabre-rattling' for when the A50 process is revoked. Up to now we have seen how the disgruntled remain resistance has operated in succeeding in thwarting the ERG's plans; this is a taste of what to expect from disgruntled Leavers when they fully realise they have been shafted and begin their guerrilla campaign.
The warning is for the EU though, not the UK, and it might only come about if Theresa is replaced by such as Boris. It is the first shots in the post (failed) Brexit political 'trench warfare' that will grip the UK and the EU, at least until the next GE scheduled in 2022
Tin hats at the ready!