It probably is optimistic, but public opinion, and government policy, can change quickly and sharply. For example, how is Greece going to fund a total loss of tourism income? Will the other EU countries bail them out at a time when they face serious pressures of their own? They could easily decide to open tourist hotspots because they think the risks will be low due to many guests no-showing (but not be entitled to a refund). Some destinations reliant on tourism might take the view if they have had a couple million coronavirus cases (including untested ones), then a few hundred or thousand more cases in a tourist population with limited contact with the residents isn't going to shift the dial.

Again, not saying this is likely, but the error-bars are potentially a lot wider.