In any case, just because every legitimate user could be a relay node, doesn't mean the relay nodes can't be Sybil flooded (attacked).
They certainly could, it's just the argument that someone providing relay services for free is suggestive of questionable intentions does not apply to i2p the same way it might for Tor. Most of the time it is simply an indicator they are an i2p user.
I had already made the point that is it is the opposite actually. I2P blinds us to the level of suggestion about questionable intentions.