Ok, but given that there are no license terms anywhere (not in the .zip with the code, and I don't think there is one on the web page), you also can't say that this particular license disallows modification. The terms are simply not specified.
Further, the very operation of this code requires the user to modify it, as described in the usage instructions. You might say that this only gives permission to modify a particular subset of the provided code, to which I don't have an answer (other than that it's unspecified).
You might not be able to fork it and distribute your modified version to others. It's not free/libre. But you can read it and modify it for your own use. To me, that's plenty enough for a project like this. As far as proprietary code goes, this is as harmless as it can get - instead of criticizing it for not being open enough, it would be better to praise it for being this open, to encourage other authors (who would otherwise keep all the code as closed as possible) to follow this model instead.
Yes, FLOSS is good and great, but it's impossible to make all code like that; in reality, where we deal with DRM, app stores, and heaps of unrepairable, uninspectable, obfuscated, phone-home activated code all around, even a bit more openness helps.
A lack of a license also does not grant you permission to modify it.
Source available software is not open source, and attempts to redefine it as such are ridiculous. This apologism for proprietary software (hypocritical proprietary software, at that) doesn’t benefit anyone.