He wasn't really trying to hide it. The ability isn't something he likes people to know about these days. Like mentioned- It's been a long, long time since he truly healed anyone.

"Semantics, I suppose," he offers, with a half-hearted shrug. "I am able to heal it from myself," or rather The Scourge is, "but it needs to get to me first. I can suffer far worse than most people can due to my unique circumstances so- Why not? I don't make a habit of it though. Too charitable and people would take advantage of it, so! Better it was kept secret."

While there is some truth to that, it was also just giving away too much to both The Empire and Insomnia. He needed to remain as unconnected, unattached as possible for things to work out right. And even if he doesn't need to be here, well- Old habits die hard. Also difficult to adjust to still is reverting back to a Noctis who barely knew him versus the one he was ready to face at the final hour.