Getting a warning 24 hours "early" that you might be infected is still useful. Perhaps in that 24 hours you were going to make a grocery trip and potentially infect a bunch of people, but now you won't because you got that early warning.

Claiming that something is useless because it doesn't cover all cases seems a little weird to me. It's a tool in the toolbox that can reduce the possibility of spread in a limited set of cases.

Using lateral flow tests requires everyone to purchase a big supply of them and use one every day. This watch-based warning just requires you to have an Apple watch and run a new app on it (which I hope they'll make publicly available), and it'll just let you know without any extra action taken. Your average Apple watch user is probably pretty likely to take the simple step of installing a new app; they're not going to buy a supply of lateral flow tests (which they probably don't even know much about or haven't even heard of) and test themselves every day.