Luis is the standard test case for how the tree scheme was built, because the tree assigned to it is almost certainly not what the name means.
The problem
The ordinary Old and Middle Irish word for rowan is caerthann. It is not luis.
Luis is much more plausibly related to lus, a herb or plant in general, or to luise, flame or radiance. Neither gives a rowan.
What the kennings say
The briatharogaim gloss the letter with phrases about the delight of the eye and the strength of cattle, readings that suit brightness or a herb far better than they suit one particular species of tree.
The rowan attribution appears in the later scholiast material, where the commentator has already committed to the principle that every letter is a tree and works out which tree each must be. Given luis, and given that a tree is required, the rowan is a reasonable guess. It remains a guess made by a medieval scholar who was reasoning from the same limited evidence available now.
Why it matters here
Because the modern divinatory meanings for this letter are built almost entirely on rowan folklore: protection, witchcraft, the tree planted by the door against harm. That folklore is real Irish and Scottish tradition about rowan trees. It simply may not have anything to do with this letter.
McManus's general finding applies: around eight of the twenty letter names are securely arboreal, and the remainder were fitted to the scheme afterwards. Luis is one of the ones that was fitted.
The entry stands, because it is what practitioners read. The chain from the letter to the meaning has a break in it, and the break should be visible.