Dogie is correct about the interpretation and unless things have changed, the metric goes back to multiple levels.
Unfortunately they don't seem to explain it very clearly themselves but my guess is that what really matters is the % of BTC in terms of UTXOs that can be clearly traced between the two parties (also it is not clear if that includes "levels" of indirection).
So if Alice was sending to Bob a total of 1 BTC but used a mixer to do this assuming that the mixer did its job correctly the % of Alice's coins that can be directly traced to Bob should be no bigger than of basically any other involved party (say 1%).
Taint is a broken concept for cryptocurrencies like bitcoin (and let's forget monero, zerocash, ...).
Computation of taint requires that you can determine with certainty the link between inputs & outputs of a transaction. Systems like mixers or coinjoin break this link. The best you can get is a probabilistic metric.