The rules say you can reveal after any step, phase, or action, and then go on to also allow you to reveal after an activation. The important thing to note here is that flipping a marker of any kind is not a step, phase, or action unto itself. Markers are simply reminders to let you know what you have and haven't done. So, to figure out whether or not you can reveal an enchantment after flipping a marker, you need to think about what it is that marker actually represents doing.
For the action marker, flipping it indicates that you have activated the creature. You are allowed to reveal after activating a creature so you are fine. For the quickcast marker, it represents that you have completed the Declare Spell step of a bonus quick action spell for the round. You are allowed to reveal an enchantment after this normally, but seeking dispel specifically prohibits revealing and overrides the normal rule.
That's the important thing here. The quickcast marker doesn't indicate that you're about to cast a spell. There is no warning step or activation for it to represent. It indicates that you HAVE cast a spell. You have used your quickcast. It is not a separate thing from the quickcast.