Assuming MWI is correct, the probability of intelligent life in this universe is 100%
The probability of intelligent life in this universe is 100% conditioned on the fact that we are intelligent, and barring the fact that some powers would likely challenge this proposition.
Assuming MWI is correct, and that by Universe you mean the larger, seemingly infinite structure our Hubble Volume is embedded in, then yes, the probability of intelligent life (other than us) is unity. Though we are still alone if they are over the Hubble Horizon.
If by Universe you mean "our" observable universe, then MWI seems to guarantee that some (possibly small) proportion of branches of this Hubble volume will have no intelligent observers other than us. Still other branches/histories of "our" Hubble volume should have no observers at all.
The probability of intelligent life in this universe is 100%
Right. Now I feel stupid for missing that.
Different objection: the amount of life isn't a poisson distribution. It has much thicker tails, as the absurd amount of life in MWI shows.
Consider a scenario in which there are three rooms. In each room there is an independent 1/1000 chance of an agent being created. There is thus a 1/109 probability of there being an agent in every room, a (3*999)/109 probability of there being two agents, and a (3*9992)/109 probability of there being one.
Given that you are one of these agents, the SIA and SSA probabilities of there being n agents are:
The expected numbers of agents is (1(3*9992) + 2(2*3*999) + 3(3*1))/(3*1+2*3*999+1*3*9992) = 1.002 for SIA, and (1(3*9992) + 2(3*999) + 3(1))/(1+3*999+3*9992) ≈ 1.001 for SSA. The high unlikelihood of life means that, given that we are alive, both SIA and SSA probabilities get dominated by worlds with very few agents.
This of course only applies to agents who existence is independent (for instance, separate galactic civilizations). If you're alive, chance are that your parents were also alive at some point too.