I was also curious about this: they mention a few times that it’s a privately owned cable, does that also imply it will be used privately (eg only Google services), or will other ISPs be able to rent capacity on it?

Most cables are privately owned, as is most of the internet actually. The telcos juts interface with each other and sell each other bandwidth on their cables.

That said, Google will likely reserve some portion of the cable for their inter-datacenter use. That’s how you get fast cross region data transfer in GCP. Same for AWS and Azure, all have some capacity reserved for data between their datacenters.

Transit is sold in bulk and peered at exchange points. There are SLAs, different rates etc. It's not much different from other kinds of resources, but I don't think it's nearly as automatized as it should be.