But did itsuno really reboot it?
Yes, twice in fact. DMC 2 was the first, DMC 3 was the second. DmC would be the third reboot DMC has experienced, possibly making it the most rebooted series to date.
To explain in further detail, DMC 2 was a reboot because of the change in Dante's character and the fact that nothing of DMC 1(or any of the other games for that matter) is even in there (outside of Trish being a playable character, but she's not in the storyline). Since we don't see a change in Dante into his surprisingly dark and quiet attitude in any part of the series, we can't say for sure that it is a part of the series outside of Itsuno's word saying that it is. For the most part, it just seems to be randomly placed in the franchise. A better term for it would be a filler, but because Dante's personality has changed it goes more along the lines of a reboot since it changes the direction the game was originally going.
As for DMC 3, if you compare it to DMC 1, they almost look the same with some major and minor differences. It was almost as if Itsuno had taken DMC 1 and rearranged so that it was the same story but with minor cosmetic differences. Then there are the gameplay changes to add on to that. Basically, if Vergil wasn't defeated by Mundus in the secret cutscene, this would be a copycat of DMC 1 instead of being a prequel.