Well, in purely military terms, technically Intel and AMD are only a few miles from Apple and their engineering corps is likely far larger. They could all march over there with broadswords if they really wanted to.
Completely off-topic, but: I think the state of the art in castle design (pre modern explosives anyway) was a star/bastion[1], since that allowed defenders to have overlapping firezones, especially useful once an attacker reaches the walls. With a circular design like Apple's HQ, as attackers get closer to the walls fewer and fewer defensive positions can see them until you can only see them from right above.
Clearly the move is to put all AMD and Intel engineers on the inside of the circle. That way they would be visible from all locations on the ring at all times.
Intel basically hit the clock speed limit and diverged to multiple cores. However, they still make x86 based chips, not ARM. They owned an ARM license for a while and got rid of it. For whatever reason, Intel felt like putting all there money on x86 was their only option. For a while they were making Atom chips for mobile, but at some point that design was hobbled because Intel has always been about the 60%+ margins on server chips. You cannot sell the cheaper chips at the same margins. It's not that Intel couldn't technically figure stuff out, it's that they couldn't see past those 60% margins.
For a while Intel's process knowledge was supposed to be better, even if the design was less efficient, but that turned out to be a mirage around 10nm or so. Intel now without a process advantage is probably never going to regain it's monopoly, and so far hasn't really transformed itself to do anything other than build those high-margin chips.
Once upon a time, I wanted to use one of the chips from a company they bought in networking, but Intel's model is to make the chip and let other companies make a product to take it to market. Intel doesn't want to make a market, just sell into it. You can see that with their attempt at TV where they stopped when they didn't want to spend money on content. So the chip I was interested in didn't get much R&D or a product and it more or less disappeared, another wasted investment.
What do you mean “how they let Apple do it”. Do you think Intel & AMD could stop them?