That's Windows-only, though. Someone recently released a "demake" with fewer levels for Pico-8 that you can play in a browser: https://www.lexaloffle.com/bbs/?tid=142638
A game jam is like a weekend hacking event where a bunch of people make small games in a short time frame and then get feedback or compared. A demake is a remake but usually for older systems or in an older style / lower resolution. In the indie game scene lots of the big hit games were first created at a game jam. No tomfoolery, they're useful words to describe things, that don't have other easy ways to describe.
Demakes can also be games that boil down the essence of their genre or original game to simpler rules. A great example of this is Footsies, which is a demake of the trad fighting genre.
Game jam - like a hackathon, for games. Groups are given a theme or conditions and a short time (eg a weekend) to make a game as a challenge / for fun.
Demake - remaking a game for a more limited set of hardware. Remake normally implies increasing quality, whereas a demake is focused on cramming as much quality as possible in a much more constrained execution environment. (In this case, a PC/iOS game to a Pico-8 game.)
Demake in modern times is often used for example with porting DOOM to (insert electronic device like a temperature sensor that shouldn't normally play games). Since the target platform has less features, it's not a re-make, it's a de-make. This could also be for more academic purposes, like "what kind of game would we make if we deleted all the guns out of DOOM".
But it comes from the old days of gaming. You could play an amazing fighting game at the arcade, but the home console had considerably less power. So the home version of that game might be very limited.
Back then we would just call that a port. Port was used to mean taking something from its native platform and putting it on something else, which was rarely an upgrade. Putting an NES/famicom game onto Genesis/master system would involve upgrades (sprite capability) and side or downgrades (sound chip differences).
Port today generally means a game has the same quality across all platforms, barring the bare facts of hardware capability. So de-make now often means a port with some intentional limitation. "I know a digital home pregnancy test can't run DOOM, but can it run enough to be playable?"
DOOM on a pregnancy test is still a port and not a demake.
A demake is not specifically on more primitive hardware but is in a morr primitive style that may run on more primitive hardware.
But it is from dialogue that knowledge emerges. Please feel free to ignore questions that don't interest you. I appreciate the question and its answers.