I would also like to know. Even HP doesn't seem to know. Because the Pro Mini 400 G9 is listed on the HP Austria website:
https://www.hp.com/at-de...

Not on the HP Switzerland page:
https://www.hp.com/ch-de...

HP's Chat Support refers to technical support on the phone. But calling them is too stupid for me now.

The March 2024 edition service manual for pro mini 400 G9 didn't include thunderbolt flex i/o in the options. I would assume it is NOT supported, even though the flex i/o thunderbolt v3 board hp webpage did mention this model.

Please ask HP support here. I bought the part because of the review below and had to return it because the plug wasn't the same. Annoying.