Yeah the SAE and metric mish mosh on these is maddening. However, i can explain why pretty easily. Everything was SAE thread, and as the U.S. automakers transitioned to metric, they would transition only as they redesigned vehicles and components so they could absorb the costs. If the tooling is churning out a current part, no need to change to metric until a redesign right. This is the reason the GMT400 gasoline engines are SAE, they were using tooling from the 1950s lol. No sense changing it, until the next generation of engines was designed.
Mopar was the same way. The Ram pickups were redesigned in 1994 and the truck went metric, however the Magnum series of V6 and V8 engines were still being made on early 1960s tooling and stayed SAE thread until the crappy mercedes sourced DOHC V8 and mopar designed Gen lll Hemis came out. Ford had the same thing with some things too. I have a 1999 Ford Explorer 8.8 rear i cut down for a project car. Everything on it is SAE thread including lug studs, and brake lines. Only thing i found on it thats metric is the bolts holding the disc brake calipers on lol.