Am I blind or did you forget a link to the full guide to getting the mount? Your first screenshot shows the full guide with the table of contents in the left side, but I don’t see a link that guide.Edit: Oh I see the link, right above the first screenshot. I was trying to click it but fat fingering my touchscreen and clicking the image instead of the link.
It appears the egg is up and interactable right now during the Amathet invasion as well. Not sure if it is a bug or if they intentionally released it to be obtainable during every invasion.