I had a couple of cats. I went on holiday, and left them at a cattery for a couple of weeks. The older one managed to open the cat-room door by pushing down on the handle, then ran up and down the long corridor until noticed by the cattery owner. Then the younger cat died, and I got a new younger cat. I went on holiday again, by which time the older cat's back legs were knackered and he couldn't jump up. But I warned the (different) cattery owner that he was an escape artist. This time, he taught the younger cat how to push down on the handle, and again they ended up running up and down the long corridor. Cats are escape artists.

I was rearranging a bedroom once, and flipped the mattress vertically, long-ways, against the wall to make room to move everything else around. Literally as I was lifting the mattress a cat started climbing up it to reach the new, higher vantage point in the room.

the GP is saying that across a sufficiently large population... you will have cats escaping (some subset of the population), and thus a feral population will eventually form.

It does not necessarily need to be your cat that kicks this off.

One trustworthy indoor cat steward can get away with it; a large enough population will naturally contain some careless people, or even surprises (hit by a bus)

PAWS in Seattle will give you a cat and it comes for a coupon for a free spay or neuter (often kittens are too young at adoption time). Of course the fee for adoption is higher than the cost of the surgery, so you are paying for it whether you use it or not.