Hey, Hi, and Hello, long time 
Makes more sense that it was a duel.
I think this is setting dependent, but even beyond that I don’t agree. Not agree is fine of course, different strokes for different folks.
- “Healing” is a lose term, as damage and “hits” to HP aren’t really physical damage (though it can be). Their HP going up can just be them refocusing themselves for the battle before them (Presence heal). Narratively this can just be them taking in a deep breath and adjusting their footing as they seem to prepare themselves. The PCs don’t necessarily know that the creature/NPC is healing or that their HP is going up.
- Healing coming from other NPCs (again, your example was a single foe) makes for more tactics and meaningful fights, b/c then the PCs can see who they need to target or disable so they don’t keep buffing and encouraging the other NPCs
- Most of the players in my games recognize that they can heal, so they recognize the same can happen to an NPC
- Now, I do agree that constant healing can have a negative impact, and it depends entirely on the situation as to whether I would have an NPC heal a lot. Also whether it was a more important fight, or just a simple one.
If this is actually true, then generally I don’t really give them stats. The PCs aren’t going to be able to touch them anyways, so its all narrative.
If you are going to give them stats and be beatable by the players, THEN, again, I have to say that HP is only 1 defense. You aren’t focusing on the right defenses, that of Guard, Toughness, Resolve, and Resistances (via boon or natural) that can be put on them.
You rolled a great 40~50? Against a normal foe that would have done a lot of damage, weakening them or out right knocking them out. Against THIS thing? Well a Guard of 30, and then a resistence to that forceful damage, you scratch their plated skin and aggrivated them slightly. That certainly makes them feel pretty epic and ancient and tough to me.
Then there is always the rule of common sense to come into play for certain things. A 50 foot x 50 foot creature, you aren’t going to be knocking it down with the Knockdown bane.
Naturally, you can give it more HP, or even, not give it any HP at all, and just record damage as it happens. Then decide that enough HP damage has been done, or that last epic attack that the player spent some extra legend points on and spent a round or 2 helping setup with the other in the party is the blow that finally fells the beast.