In the same sense that you can use third-party material or homebrew as resources, sure. If they wanted you to treat older-edition lore with the same weight as current lore, they wouldn't have made a policy statement that implicitly walled it off. They want you to privilege the use of 5E as the only current canon.
Did Perkins say that as an official recommendation of Wizards of the Coast, to use older-edition material as canon for the current edition? Or just as an offhand comment about useful resources?
Then why did they publish a canon policy out that says older-edition material is only canon for its edition, and only 5E material is canon for 5E? A policy that encouraged use of older-edition lore with current material would look rather different to what we got. Instead, they strongly suggest that's not the way to go.
Greyhawk has appeared in 5E, in Ghosts of Saltmarsh, so that's no longer an older-edition reference. The Mystara reference is interesting, though... Mystara confirmed?