There are a bit too much circumstancial evidence on this part to be frank
Spoiler for episode 6:
-Both men were obviously not calm and were arguing.
-Mikasa was not treated in any shape or form (be it a blanket, jacket whatsoever)
These 2 points immediately nullify the presumption that these 2 men were benefactor right form the get go.
I personally think it takes more suspension of disbelief to think that "2 men unrelated to the murders are conveniently close within the same forest, with an injured girl, having a not so friendly chat".
That by itself isn't a rock solid evidence, but that's a glaring chunk of convincing cirmcunstancial evidences, to which is added by the hostility of one of men against Eren.
Frankly, you have to consider the context of the situation: within the same day, the said boy found an asian girl, being isolated in a cabin, unconscious and on the ground, with 2 grown men being flustered, within a close perimeter that the child himself could reach without taking too long.
Please note as well that Eren state "もうわかった", which means he made his conclusion by the time the first guy approached him with hostility then with an obviously failed empathic approach.
From what it is worth, I frankly think that the assumption required for Eren to make such conclusion doesn't need that much of suspension of disbelief, contrary to the idea that "these persons are there by chance and are unrelated to the incident that happened moments ago".