Model M’s all use a membrane and a plastic barrel frame. https://deskthority.net/wiki/IBM_Enhanced_Keyboard

The capacitive keyboard was the Model F. Instead of the plastic barrel frame, it uses separate plastic barrels held between two metal sheets, and the little conductive plastic flippers trigger capacitive pads on a PCB. https://deskthority.net/wiki/IBM_Model_F

There are pictures of the insides of a Model F XT keyboard here, http://vintagecomputer.ca/ibm-pc-model-f-keyboard-type-1-vs-...

Here are pictures of someone modding a 122-key Model F https://deskthority.net/viewtopic.php?f=7&t=11102

This source here claims that the model M switches are capacitive - https://pcper.com/2014/12/mechanical-keyboard-switches-expla...

“This source here” is wrong (as was pointed out in the comments below in 2014). It is just misquoting Wikipedia, which it links to: “In a Model M, the electrical contact is a membrane sheet similar to that of a modern dome switch keyboard. On the older Model F design, a capacitive contact was used instead.”

Go ahead and take a Model M apart. I guarantee you’ll find a membrane sheet.