What the GP is saying is that the T2 is effectively “the processor” of Macs it’s in, in the sense that it’s performing the function of a System Management Controller, managing early boot, etc. The Intel CPU in these systems is reduced to being an “application processor”—much like in ARM systems where boot and IO are handled through the GPU.
(This is why you can’t install OSes other than macOS on these devices. It’s not because of DRM or anything—though that’s an aspect of it. Rather, it’s because the T2 is in control of boot, and no other OS has had a second-stage bootloader written for it that will load onto the T2 chip to do hardware bringup from that side.)
Maybe “bridge” is the wrong word here—though I can see the meaning. A precise term, I think, would be “polyfill.” The T2 is the place where Apple is putting 1. its IP-core customizations that it can’t just plop into the Intel CPU, because it doesn’t control the Intel CPU; and 2. its top-level “BIOS-alike”, allowing them to bring up the hardware in a way they control, rather than having to convince the Intel CPU to do it in a way it understands.
The T2 and the Intel CPU acting together, roughly simulates an architecture akin to that of an ARM Mac (as with any other polyfill, where the old runtime and the polyfill acting together roughly simulate the capabilities of the new runtime.)
The interesting thing about this, is that it’s likely that because the polyfill roughly simulates the environment of an ARM mac (where these IP cores are accessible directly through the CPU, and where hardware bringup is always done in Apple’s preferred fashion without having to trick the Intel CPU into it), the drivers written for Intel-plus-T2 macOS, are also likely able to be run with only small changes, directly on ARM macOS.
That’s why the GP is calling the T2 a “bridge”: it’s a half-step across the chasm between the archtectures; an intermediate architectural target that allows some but not all of the work of developing ARM macOS to be done before any Apple Silicon ARM platforms like the A14Z were actually available to be developed on.