Because Uilleann survives only in the grammatical tract tradition and not on any of the roughly four hundred surviving inscribed stones, its evidentiary footing differs from the twenty core letters in a way worth stating plainly. The stone corpus fixes the original alphabet's shapes, order and, largely, its sound values through direct physical attestation. The forfeda rest on manuscript copies made centuries after monumental ogham had gone out of use, by scholars reconstructing and extending a system they had inherited rather than one they were actively cutting into stone.
That does not make Uilleann fictional. It makes it a different kind of fact, a scribal solution attested in a specific textual tradition rather than an inscriptional one, and it means claims about its exact stroke form, its precise sound, or its supposed symbolic weight should be weighed against how thin that manuscript record actually is. Where later popular ogham material asserts a confident, fixed meaning for Uilleann, the honest position is that the letter's core, sourced facts are its sound gap and its honeysuckle gloss, and everything built on top of those two points is interpretation layered on by later readers.
Sources A Guide to OgamDamian McManus with Auraicept na n-Eces: The Scholars' PrimerGeorge Calder (ed.)