The "Egg Prime Directive" is the notion that you can't tell a person you suspect of being trans and unaware of it that they are trans. It takes its name in likeness to the Prime Directive of Starfleet in Star Trek, where members of Starfleet are forbidden to interfere in the development of pre-warp cultures.

And that's where the misunderstanding starts. As such, I agree with not telling someone who hasn't had their lightbulb moment outright "Hey, bet you 20 that you're rocking thigh highs and cat ears within the year?"
Because being trans is a claimed identity. You have to claim it for yourself. It can't be given by anyone, not by peers, not denied by gatekeepers. It can only be taken by yourself. I'm not in someone else's head, I don't know how they're feeling about things, I simply can't tell them, because I don't know.
And the harm might be, that lightbulb moment might be in the making, and is waiting for the right headspace. Which is a delicate balance of situation, knowledge, and state of mind. Throwing that idea in there when it's not the right time might well lead to deeper rejection - food looks only good to the hungry, knowledge and opinions only to those who ask.

Personally, in retrospect, there were only five points in time where someone telling me would have lead to a positive outcome, aka, help me: when I was 4 and wrestled with what to do with my new found knowledge in face of an either bigoted, or at least complicit family; at 15 when I was twiddling the broad magenta strand in my otherwise black dyed, shoulder length hair one afternoon; at 20 when I sat lost in thought on a bench thinking about why I found a fellow student at uni so strangely attractive, but felt absolutely no distress about not talking to her; at 28 when I bought my first high heels and didn't know what to make of it; and finally, at 37 when I hiked along the beach of Whitepark Bay in the rain after a storm, listening for answers in the surf.
At every other time, the notion that I might be trans would have gotten dismissed out of hand, and as such, would have been deleted as possible answer for quite some time after. Would you feel confident hitting a person at the right time with a revelation, if you have to imagine those odds? I don't.

But a lot of people seem to take the Prime Directive parallel to really odd lengths. Sure, you don't walk up and say "You're trans". But you are absolutely allowed to talk about your experience. Discuss. Share joy. Live the best trans life right in front of their eyes.
You are absolutely allowed to say, "you'd look cute/handsome/radiant with that [nonconforming clothing]". You can absolutely suggest to play with the expectations of their gender role, bend them, break them. You can tell them, "why sure, I'll help you with some make up!"
You can absolutely, completely, totally enable every genderfuckery they deem necessary on their way to their own lightbulb moment.

It's not because it's sacred, or a mysterium, or whatever. It's simply because, in the end, you're not in their head, and you don't know what's going on in there, and their life. You don't know if they're receptive. If they're looking up to you, there's power dynamics to consider. You don't push this in their hands. But you can do your damndest to put it in plain sight in comfortable reach.

You don't have to hide anything. We're not Starfleet, we don't have to hide behind holographic blinds.