I can guarantee their TAM is >$108M / yr.

I am involved in two websites that get a lot more than 10k visitors / month. Neither are listed in Quantcast. Using (crummy) traffic estimates based on Alexa ranks, I'd wager that there are at least 600,000 websites that would qualify for your >10k monthly visitors mark.

But I suppose it's all a moot point: They convinced at least one VC to invest, and that's all that matters. (Having used their product in the past, I probably would've invested too given the opportunity. Their product is pretty slick and saves a lot of time compared building your own A/B testing framework.)