That depends on what you mean by “reboot”: certainly, Legion fans would quarrel with any claim that Zero Hour wasn’t a reboot; and Infinite Crisis’ changes to the main Earth’s continuity were arguably every bit as drastic as what Zero Hour did. The only real difference is that Zero Hour and Infinite Crisis didn’t make drastic alterations to the current state of affairs the way Flashpoint did; they primarily focused on changing the backstories that lead up to the current state of affairs.
But then, even Crisis on Infinite Earths didn’t go as far as Flashpoint did; to the extent that it was a reboot, the reboot was spread out over the subsequent decade. Justice League Detroit was still in continuity when Millenium forced them to disband and ushered in the JLI; the Green Lantern Corps started out featuring a team on Earth that clearly remembered the pre-Crisis Multiverse, which didn’t change until four or five years later with Emerald Dawn; Hawkworld lead to one of the messier aspects of the decade-long reboot process. And so on: in many cases, the titles continued on right from where they had been pre-Crisis, at least for a while — much like the way the changes brought about by Infinite Crisis tended to take hold over an eight-year period.