If I hear a phrase being repeated by politicians ad nauseum, such as Theresa's "let us be clear/in the national interest/a deal that delivers..." or JRM/Raab/Javid et al's "antidemocratic backstop" it makes me feel sorry for the poor saps. Imagine doing a job where people tell you what phrases to go out and parrot that day/week, and imagine not having the backbone to refuse. This rhetoric is like a verbal version of the 'Tory power stance', if you remember that particular bit of ridiculousness.
I agree with you about the jarring of hearing someone repeat the given phrase, absolutely.
But whilst it's obvious that BJ is repeating the phrase, when it seeps into becoming part of the natural speech pattern of commentators it becomes 'normal'
For example, I've noticed that news presenters are saying things like "for Boris to get Brexit done, he needs to pass xyz" or "if x blocks y, then there's no way Boris will get Brexit done"
The 3 word catchy slogan can really work if done properly, but is really really obvious when done badly (TM strong and stable for eg)