"'S e cèic a th' ann."
Translation:It is cake.
It can’t. You can never use a noun phrase (and cèic a cake definitely is a noun) as the predicate of the bi verb (and tha is a present independent form of bi).
’s e cèic a th’ ann means literally it is a cake that is in it
As you can see the predicate of the verb tha here is ann in it which is not a noun (that’s why it works here). In
*tha e cèic you’d try to use a cake as a predicate of tha and you can’t.
It could be also said as tha e na chèic, lit. it is in its cake (which is another idiomatic construction for it is a cake; here na chèic in its cake is the predicate, again a prepositional phrase, not a noun).
Or as is cèic e which is straightforward it is a cake – but this is a high-register poetic/literary language. In a normal conversation you’d say ’s e cèic a th’ ann.
(there is one exception when a noun-phrase predicate works in Sc. Gaelic: the phrase X a tha seo this is X, X a tha sin that is X, instead of longer and more regular
*X a th’ ann seo, *ann sin; but still you don’t say directly *tha seo X, you have to say is e seo X or just seo X for this is X without a relative clause)