It appears to be about 3 things:
1. The assets (graphics, music, etc.), as they were all taken from commercial Pokemon games -- and it appears to be VERY extensive (across several platforms/releases, all the way back to original GameBoy from the look of it),
2. A modification to RPGMaker (commercial PC game making software), which includes scripts/all the necessary bits to make your own Pokemon-like game via RPGMaker. In other words: the community likely did all of this work, and the end result was similar to the existing commercial Pokemon games,
3. Probably as a "convenient addendum", use of the word Pokemon (with accented e) which IIRC is trademarked or copyright, i.e. branding violation.
* Here's a tutorial video demonstrating its use and some of the assets.
* Here's a video demonstrating what stock results could look like.
Youtube is filled with videos of people showing how to make your own "whatever" using Pokemon Essentials, though the goal is to make something that's Pokemon-like.
To clarify on #2 above: it does not
appear to be a romhacking-esque tool that let you tweak the entire engine/assets/etc. in an existing Nintendo game (think ROM) and rebuild a working binary for an existing system (present-day or classic). There are tools like that already for actual Nintendo games though (Zelda, Metroid, and Super Mario World come to mind). The ones for SMW are extensive, and I imagine it's just a matter of time before Nintendo does something about that.
Take a look at an archived copy of their Wiki
-- it's all about how to make whatever type of Pokemon-esque game you want, down to what all of the individual .txt files and .pbs files are for and their syntax/usage.
The actual .zip file of the assets and RPGMaker mod wasn't on the Wiki -- it was actually on some file hosting service which now returns basically no-such-file. The last version released appears to be from October 2017, so almost a year ago.
Key point: there are tons of games out there that are Pokemon-like in style... just that none of them are using ripped assets from actual Nintendo Pokemon games. That is the major difference.
Are people here unaware of what happened to Rachel Simone Weil (who does NES homebrew of sorts
-- I assume everyone knows her by this point) and her "Pokemon 7" homebrew? Nintendo issued her a DMCA takedown
back in late 2016. It wasn't a Pokemon game for the NES, it was a completely unrelated thing that contained Nintendo-owned assets. Look at the tweet. Hell, I even mentioned this before
on this forum!
Respectfully: people arguing against my points here need to step back and think about what this site is for
: it's for general knowledge, homebrew, and gamedev. If Nintendo is willing to issue DMCA takedown for a Wiki -- and that is what they did, read the news articles, it's not just about the .zip file -- that contained "howtos" on how to use said .zip file contents in RPGMaker + make your own Pokemon-like game -- and also for homebrewers who used Nintendo's own assets... yeah, think about it. The only difference between "us" and "them" (the Pokemon Essentials site) is that there's little-to-no Nintendo-copyrighted assets on the Wiki. Screenshots are "fair use", I can't argue against that, but when you put those types of things up on a site dedicated to development, the situation becomes a bit different.
So I'm back to stating my point: we need to be careful about attachments/etc. going forward, and I strongly argue for review of ones that are already here. I've been saying this stuff for years (actually *decades*, from my own personal experience with Nintendo back in the 90s with my SNES docs), but people seem to think I'm being overly paranoid or that I should wear a tin-foil hat or something.
I'm not saying "THEY'RE COMING FOR US!". I'm saying "let's not give them a reason to come". There's more than enough evidence that yes, they obviously do care about even the smallest things.