It’s nothing to do with the reliability. It’d be like finding out that Apple had shipped a laptop based on Dell internals - whatever the results, it’d indicate something tectonic had happened in Cupertino. It’s just not something the company would have even considered a decade or so ago.

The BMW Z4 2018 and later models shares platform with the Toyota Supra 2019 and later. The platform, at least according to both brands marketing, was co-developed, and it is manufactured in Austria by a 3rd party that also built cars for other brands.

And no, Toyota doesn't make the most reliable cars. It might have been true for some period of time, but isn't true for every model and every year (even Corolla, often suggested as a very reliable sedan, has some versions that are in fact infamous for their tendency for oil sludge buildup that result in engine failures).