Seems similar to how there were claims you could move your project between cloud providers but that was never true because they had some many hooks into native services.
models are different, they are easy to swap out because they all use the same api schemas and are converging
however the harness may very well end up as you describe, which for many is the same things as a vendor model family for those using closed models, the open model ecosystem is much easier to swap more of the stack in