It all depends on the license, ultimately. Where source code is available, as it is with Air Manager because of how instruments are distributed, then anyone can modify and tinker but unless the license permits you to distribute such modifications then merely attributing the original author doesn’t grant any immunity from copyright infringement claims. That’s why properly-written open source licenses are so popular; they work both ways to protect the original author’s rights and the rights granted to everyone to modify and re-distribute. Distributing source code without a license file indicating the rights you’re granting or reserving is asking for trouble, I wish more AM creators did it. All you really need is a LICENSE.MD in the root of your repo and maybe a note in the human-readable files about ‘distributed under <name of license>, all rights reserved’ and then everyone’s clear about what they can and can’t do with the code.
FWIW, any instruments I develop for AM will be released as open-source with a BSD 3-clause or MIT license which would let you do pretty much whatever you wanted provided you credit me.