If his explicit aim is to "cripple" trade between NI and GB, then of course that would be bad faith. Do I choose to believe an unattributed quote in a very pro-Brexit, pro-Govt newspaper over the written facts? Not without further proof.
By contrast, what the UK Government is doing is unarguably bad faith - written in proposed legislation, with a Secretary of State admitting this is a breach of international law.
It's reported in more than one newspaper. In my view, it would be justification for what the UK is doing.
Even if he did not mention ban, but threatened to use the EU's discretion to put tariffs on all UK goods coming to NI, that would be bad faith too, since the discretion was intended to apply to "at risk" products and although this is not defined, it was clearly intended to be an exception not the rule.