My bolding. This is the key.

I suspect that this is why the website is rather vague since whether someone intends to permanently import something is a matter of judgment for the individual officer.

However, in practice, it's all entirely irrelevant. They stop very very few people and, AFAIK, those that are stopped are singled out through intelligence-led information, with perhaps a handful of randoms thrown in. I would say that only half the time that I walk through Customs (perhaps 25 times a year) is anyone currently stopped, and, for the information of the OP, literally hordes of people are walking through at the same time.

Now, if the OP is coming from, say, China, with a suitcase full of phones all in their original packaging, that would be a different story. But if arriving with one used phone and one used computer, both of which are obviously currently in full time use, absolutely nobody is going to question it.