I could be wrong, but I thought that the precedent was if the song was different by 12% then it was kosher. Generally people remixing will use a lot less than that so as to stay out of the risky zone.
And seriously, if they remixing the DF song for the waiting music... That is hilariously amazing. I mean, THAT is a stretch for referencing. If that music is in fact DF music remixed, I will claim 100% that the Bay 12 and Swann being a miner is a DF reference, because that's just finesse. Haha
12% is a bullshit number. There's no hard and fast way to measure percentage-based "similarity", much less a rule about 12%.
Fair use is real, but it's based on four factors:
1. the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
2. the nature of the copyrighted work;
3. the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
4. the effect of the use upon the potential market for or value of the copyrighted work.
It passes 1, but just barely, as it isn't a commercial (here meaning for-profit, in a way that enhances profit, etc) use.
It doesn't really pass 2
3 is what you were talking about; it's only one factor, and it's more about how important the portion used is to the work as a whole, rather than an arbitrary number.
4 is more or less passed; can't argue that their use diminishes the value or market for Toady's work.