Always took it at face value, meaning the hit itself. "Ten years ago", Solozzo wouldn't have been able to take a shot at the Don of the Corleone Family and gotten away with it, unless it was planned as a suicide mission - which, given Solozzo's ambitions, it would never have been. No point at all, beyond some sort of vendetta.

Had nothing to do with Vito's own instincts or abilities, merely a reflection upon how the situation had changed - ten years earlier, none of the other Families would have dared (or even wanted) to upset the status quo. A hit on Vito would have done too much damage to their own fortunes.

Solozzo's actions, and his ability to get away with them, are meant to be an indication that the peace that the all-powerful Corleone Family lorded over was at an end. The final part of the first film is about exactly who was upsetting this peace, and how the Corleone Family dealt with this upset, which they did.

That said, the circumstances around the hit on Vito are also something of a commentary upon how complacent the Corleones may have become - the Emperor of New York is shot almost dead while buying fruit from a street vendor, with only his weakest son as a bodyguard, one who fumbles his gun in a pathetic and worthless attempt to defend his father.
There's that shot where Vito suddenly becomes aware of the danger, but this is a man who has had men trying to kill him since he was an infant. His offspring, born in the land of opportunity, are all too stupid, weak, straightlaced or hotheaded to deal with a real war.

All but one, of course.