The ambiguity of the acronym aside, the M in JAMstack emphasizes how content is delivered, instead of what format that content was in prior to pre-rendering. Content on the JAMstack can effectively come from anywhere, but as long as it’s pre-rendered it counts as “M”.
If you’d like to dig into the M in JAMstack, there’s a fantastically written post by Brian Rinaldi that might be of interest. Check it out over on his blog: https://remotesynthesis.com/blog/m-is-for-markup
And to start digging into building the Js, As and Ms in JAMstack, https://templates.netlify.com/ is a great starting point.