Using a T-Mobile sim from my iPhone X in my iPad Mini 6.

Thanks for the update. The Google Fi sim I just got works in either my iPhone phone or iPad but Google Fi doesn't yet support 5G on iPhone/iPad.

Nope. A phone sim will only work in a phone
and a tablet sim will only work in a tablet.

If you go to the tmobile section of the reddit board there's people posting that you can't and if you try you get the speeds in your original post.

Consulta: https://www.t-mobile.com/support/devices/apple/setting-up-non-t-mobile-apple-devices

Based on this web page, I expected that my iPhone sim would work in the iPad.

The Tmobile phone sim isn't provisioned to be used in a tablet which is why your getting those speeds.