In the context of the manga as a whole, Boros is an unprecedented enemy/foe for Saitama.
Incorrect, what it says is that he is an unprecedented foe in general,
not specifically to Saitama, and the fact is that it is in the same sentence in which it states "what is the earth's fate?"
When the statement says "What will be the earth's fate in the battle against such an unprecedented foe!?" it by default directly connects Boros being a strong enemy to the fate of the earth, meaning that most likely, because Boros is a strong enemy, the fate of the earth is in question.
And to get into the nitty gritty of it, why exactly would this be such a miniscule chance that a possibly rating wouldn't work for it to begin with? In your eyes, is the threshold for a possibly rating exactly 50/50 and anything below that is an impossibility? After all, to many people I've seen, saying that the battle decides the earth's fate is more than sufficient enough to convince them, let alone just acknowledging the qualifications of being vague but notable, even despite chances being unfavorable
Likely
Should be used to list a statistic for a character with some basis, but inconclusive due to the justification being vague or non-definitive. The probability of the justification in question for being reliable should be favourable. This term should be used sparingly.
Possibly
Should be used to list a statistic for a character with some basis, but inconclusive due to the justification being vague or non-definitive. The probability of the justification in question for being reliable should be notable, but mild. This term should be used sparingly.
That being said, I did suspect that this one scan alone would be difficult to push through, so here's another proposal.
I would like to hear your thoughts on the idea of just gathering a few pieces of supporting evidence and trying this thread again. With the added context of Boros' "strongest in the universe" statements, as well as Saitama's expression and the repeated times he calls him strong,
as well as that time Murata called him the "heavyweight champion of the universe", I believe we could work something out better than just this one statement, and then the possibility would become notable enough perhaps.