What Mega Man's talking about in the first panel is the simple fact that, to him, he's done all this before. Remember, this Mega Man is from the present, and from his perspective, he was around during this time period and had to fight all of these enemies then. And now he's apparently doing it again.
Of course, this brings up a rather paradoxical question. If it's now the Mega Man from the future that fought all of these battles, how could he remember fighting them in the past in the first place? Since now, thanks to time travel, it wasn't actually him doing this the first time, it was the Mega Man from the future, right? So how could this Mega Man possibly remember doing it?
Well, one possible explanation is that time is linear, and the past that this Mega Man remembers is not the same past he's currently reliving. By traveling through time, Mega Man has created a brand-new timeline, and anything he does has no effect on his past timeline. Though, that doesn't make any sense with respect to what we've seen in the present, what with everyone getting wiped out by the temporal shockwave. Further, if nothing Mega Man (or Bass) does in the past affects the future, why bother fixing anything in the first place?
No, the much more likely explanation is simply that Mega Man's memories of this time are false, and it wasn't really him that did it the first time either. It's always been the Mega Man of the future doing these things, even if no one knows or remembers it in the future. In fact, we later see Dr. Light implanting these false memories, if for no other reason than to fix
this plothole the timeline.