It ends up this isn't hotfixable. It's fixed for 21.0.3, which is coming soon.
There is a workaround, if you don't mind mucking around in your install directory. If you go to the INSTALLDIR/plugins directory, you can unzip the com_slickedit.base.zip file that's there, and then move the zip file out of the directory. (once you're done, the plugins directory should just have a com_slickedit.base/ directory, rather than a zip file). This will work because the bug is in how the files are traversed when the source is a zip file.
That workaround is probably why we missed this was broken a second time - during development, those directories are unpacked; they get packed up when we build installers. I'd bet I ran it on a dev install when I was testing it, and didn't realize it.