I'll just play Devils advocate here. Debating this will largely always end into semantics. Grammar can be at hand hugely here. Remember I'm not stating a view here I'm just running through each point.
1. You could say that the only way Superman knows that the entire Zone is in trouble is because he can hear across a dimension that is seemingly boundless. Which if seen that way would also imply his abilities to hear things is basically incalculable across any distance.
If that is the opposite and he can't hear that far (which wasn't 100% clarified) then you could say Superman is making an assumption which if you wanted to you could say is based on a well thought intuition.
2. On the other hand you could say that the scan itself says "Every creature must have been alerted to this". In language this doesn't mean they did definitely, it just implies that Superman thinks the battle is raging so much that everything inside the zone must have been able to hear it.
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He can hear Earthquakes in the distance. The term distance is never clarified further. Saying the zone is trembling again,
Again if we were to assume that the zone was trembling how would we assume this is true. One hypothesis is the fact that in this case Superman can hear across the entire spectrum of the phantom zone. Which is itself a feat that is very impressive. But if that was true then he wouldn't have said "must have been alerted". He would have said "Every creature inside the Zone has been alerted, chaos will further descend on the Phantom Zone".
3. Insomuch as someone might say something like "The world stands with you", "The world stands in amazement" or any other variation of them pithy lines. Nobody actually does know if them things are true. Can they really know if a member of an undiscovered tribe from Papua New Guinea is rooting with lets say Buzz Aldrin to have success. We know that this is in reality hyperbole. But it makes for an extremely impressive statement.
Superman I don't think is being hyperbolic. There would be little point in doing so, But if he is not being hyperbolic does it mean that the entire Phantom Zone is therefore trembling? Put simply it doesn't, even if you are on the side that believes that to be fact. It doesn't give you a concrete answer, one might want to say it is a reasonable assumption based on what he can hear, feel (vibrations etc.)
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Like I say there isn't really a way for anyone to be 100%. I don't think Bendis was deliberately trying to write the story to be clandestine except to only a few people who are initiated who "know the truth". Nobody knows the 100% truth.
The only way you will know is if someone contacts Bendis and asks him. Like the time when I asked Dan Jurgens if Superman had flown from the edge of the Universe back to Earth or just from Saturn back to Earth if anyone remembers that. And it was revealed that he did in fact fly from the edge of the Universe.
You really need to contact Bendis to find out the the exact truth. Because it isn't a definite statement either way. Not intentionally made to be vague. It is just as it would be any humans interpretation of an event that they are thinking of. If it had been Morrison writing it he would have worded it differently. If it was Jurgens he would have worded it differently. For better or for worse. Which you would then be met with the same hypothesis. Ultimately comics aren't created for battleboards (not saying anyone said that), they aren't going to say "This fight made the whole phantom zone tremble because of how powerful we all are". Because it would sound like a robot talking (which Bendis has been guilty of doing in the past). Like in a lot of poetry things are written often in flowery language. This is just a regular version of that.