The etymology reported here by Eustathius, who does not subscribe to it ("according to some people") is indeed weird. Ite relates correctly καρδία to κῆρ, but then accounts for the end of the word by the poetic use of anastrophe according to which a preposition can be found after its regime. This reverse order, frequent in Homer and imitated in post-Homeric poetry, is then transposed to a single word: since prepositional governing compounds in Greek rely on real prepositional phrases (ἐναλίος from ἐν ἁλί), a lexicographer assumed that a compound could also come from a prepositional phrase displaying anastrophe. This etymology requires a further formal manipulation, the change of [ē] to [ă], but the correspondance between η and α was so familiar to Greek scholars that it was taken for granted. The explanation is semantically bizarre: it seems to imply that the καρδία is defined, not as the organ, which is κῆρ, but as the energy contained in the organ and running through it.