It must be a pain in the ass to fix when people kept sending the devs errors that resulted by using "external tools"/ 3rd party apps/ modifying the game files that is not intended by the dev.
Also it seems there is some degree of "encryption" (self created drm maybe, which is easily bypassed as evident by the game being here) which may be causing some of the performance issues. Also I've read another user here state how the game is very "uncompressed", and the dev is planning to uncompress the game further (the file size would increase at the minimum).
Dev is blaming a lot of the bugs on Mtool...? Can anyone confirm?
He isn't exactly fully blaming, but it's kind of hard to fix the game when people kept sending him errors caused by a 3rd party app/ modifying the game files in some way. It makes fixing the actual bugs present in the game harder to do since he is also presented with bugs not present in the "base game" that will not occur without modifying the files/ using 3rd party apps. It introduces new bugs which are not caused by the dev and mixes new variables into the bug fixing equation.
I'm not saying that the dev is great at fixing bugs/ optimizing the game in any way (given that the game took 6 years to release and it still is a resource hog, possible memory leak issues, crashing issues, performance issues, etc) but reporting bugs not caused by the dev would only take time away from fixing actual bugs.