You say you think you know what arch means, but just to be sure: it's not the noun meaning the round or pointy feature of buildings. It's an adjective meaning 'playful, mischievous or cunning' (we might refer to someone's 'arch sense of humour'). Mottos and slogans are very short and memorable phrases associated with schools, cities, businesses or political parties or candidates, but they don't really tell us much. McDonald's slogan 'I'm loving it' doesn't really tell us much about the food or the company.
This person doesn't like the short, straightforwardness of (whatever they are talking about). They like the discussion to be more playful or cunning, and possibly longer longer than most mottoes or slogans.