[Avodah] Machshirin 2:4

[Sholom Simon]

Chevre,

In Machshirin 2:4, the Mishna talks about a case where one is 
laundering b'gadim in tamei water.  The footnote (Reb Artscroll) 
discusses why somebody would ever do that, and they note that: "A 
rishon doesn't have the capacity to convey tumah to people or 
utensils (including garments)," only food and other liquids.  OK.

We know that's true with a rishon in general, but we also know that a 
_liquid_ rishon _is_ m'tamei people or keilim.

And, in fact, in Machshirin 4:10 the Mishna warns about accidentally 
putting tamei wet logs into an oven, davka for the reason that the 
wetness on the logs might be m'tamei the earthenware oven (because, 
d'rabbanan, tamei liquids are m'tamei keilim).

So, what am I missing?  How do we explain 2:4?

-- Sholom