The poultices would essentially be cure wounds spells (or other medical effects) under a different name.
The thing is is that at the end of the day when you're proposing a healing ability, magic and non-magic are going to look similar mechanically and you can't really avoid that without just deliberately making one or the other convoluted, or denying the possibility to one. Ie, no healing magic, only real medicine works.
Like, I might decide to say that healing magic doesn't work on an instantaneous heal 3d4. Instead, may be it works by rolling 1d4, and keep rolling a 1d4 everytime you roll a 4, adding the total to the amount healed, but any 1s you roll subtract your last roll and end the attempt.
Or I might make poultices work over time, healing a point every hour, or I might devise some complicated rolling scheme to distinguish it from instantaneous magic.
Thats all kind of a waste of time, and ultimately, I think the importance of different types of healing having diversified mechanics or effects is being overstated. I wouldn't draw an analogy to the Alchemist because its issues lead to the entire playstyle just being a reflavor of another class, whereas Poultices, if you're being reductive, are a reflavor of a healing ability that may or may not have an equivalent in spells.
Plus, the thing about Rangers is that they should be approached from the POV of being uncanny. When Aragorn sticks his ears to the ground and can tell where a group of Uruks are and how many, hes basically doing something that may as well be supernatural to us, but he isn't waving his hands and saying some funny words to do it, and it could plausibly be a learned skill in his world.
That is how a Ranger should be looked at. The supernatural elements should be an extension of otherwise normal skills, and if we understand that, we shouldn't be afraid or concerned with emulating "magic" if we extrapolate this for 20 (or 30 in my case) levels of development.
There is also the question of why only rangers know this secret to poultices,
Funnily enough I had the same question and as far as my game goes that may well change. Particularly because all 4 of my Nature classes, which includes Rangers, Druids, Hedge Mages, and Beastmasters, already share the same core Terrain mechanic, and Id really like to be able to expand on poultices with it.
In that case, Poultices would be a default core to the Ranger, but the other three classes would have the choice of whether or not to take that ability chain.
And this is also growing in my head because I've been putting off wherher or not I want to include healing magic at all. My magic design technically doesn't support it insofar as the lore goes, and inadvertently the classes Ive designated as having a more "Healer" bent, which includes the Ranger, don't actually derive their healing abilities from magic at all.
At most I actually think I might include it as a possibility for Improvised Spells, which is where most utility uses for magic already have to come from, and any more conventional "always works" healing spells would be even more costly and difficult to get a hold of than their utility equivalents.