Google don't want to "break the iMessage stronghold". They want to crack open Apple customers' messaging so they can feed it into their advertising systems. WhatsApp is useless to Google for that.
They don't make any money or sell any ads via text messages with Android users, why would they suddenly with iPhone users?
I mean they clearly believe that they can do that by themselves in the US market but WhatsApp has the advantage of being able to lean of expatriates and people with contacts abroad as a steppingstone.