Yes, Marvel Legacy is a continuation of the Marvel Universe. It's not a reboot.
But returning characters like classic Steve, Odinson Thor, Logan, and Tony Stark, etc., is a "shake up" and "change of direction". It's giving certain fans what they say they wanted - a return of the original heroes. It's just happening as storylines naturally unfold, not in an abrupt, all-at-once, across the board return.