Well actually I think it's probably a pretty smart move. It's a small ISP so it's a doddle to migrate systems (relatively) to their own billing systems for example. But more importantly they have all the links and processes in place for dealing with BT openreach and the like. Throw in a sizable chunk of marketing money to get customers with a massive advertising campaign. So three can now start to offer stupid broadband deals to three customers not to mention playing BT at their own game of free wifi. And three are finally sorting out wi-fi calling so maybe not entirely unrelated.
So daft, probably not so much. It's just not going to happen overnight but I'd say there aren't going to go down without a fight. And given how they changed the mobile market (a bit like orange did all those years ago too) I'd put money on some market disruption coming, or at least an attempt.
How much or how successful time will tell. Also, their customer service can suck badgers balls sometimes so they may want to work on that too before trying their hand at broadband.