You have read Benny's own words on this, haven't you? :) (Your use of the word "ensures" made me wonder.) http://www.fluentin3months.com/defining-fluency-to-achieve-fluency/ So, what is your own definition of fluency? :) As someone also living away from their homeland, I find that while the opportunity for immersion is there, you don't learn the language by magic or osmosis. It's quite easy to build your own isolation-bubble, avoid the language and effectively sabotage your progress. Even with native speakers to talk to, without some real guidance you're just pecking at crumbs. I've had students who had spent a year in the UK or the US, and were still struggling at elementary level. These were the same students who thought it was OK to speak their own language in English class. No surprises there.

The real answer to this question begins with another question: "What is fluent?". I think it is possible for the average human being, with the right resources, the right training, the right dedication, to become high conversational in a second language in three months, but not fluent. To me, fluent is at least a C1 under the European framework, which is a much more useful measure of linguistic ability.

what language tests did Benny Lewis pass learning the language during 3 months?

I think no, no matter how immersed in the language you are. Unless you're a genius. Just picture the whole vocabulary of that language and the whole grammar with its rules and exceptions of the rules, and idioms and everything... I don't think that I would even have the time to utter every (relevant) word of that language once in that short period of time, let alone to know its meaning.

I don't think you can be fluent in three months. For me fluency means being able to read, write, and speak comfortably, and being able to engage in complex academic or professional activities. Basically what is defined as level C1 by CEFR is fluency, in my opinion. I don't think that level can be achieved in just 3 months. However, if we define fluency as being conversational and communicative without any noticeable pauses, I guess that can be achieved in 3 months after 24/7 immersion.