I don't think that holds.

Even if you don't coinjoin yourself how certain are you that the previous person didn't? What about the guy before him? If it's only about the last step then just doing a 1-hop to yourself solves it. If they check n steps then you just need n+1 hops.

In reality the more users coinjoin the more all coins become "tainted" and then trying to discount them is pointless.