The Appearance of Fricatives in Anc. Macedonian

Vasil Gligorov wrote
> However certain recent studies, outside the aforementioned mainstream view,
> point out that the sounds represented by Δ/Θ may already had values of  ð/θ
> in xmk even before those changes occurred in Classic Greek and not as is
> mostly encountered in the textbooks,during later Koine period
Perhaps I'm being slow here, but how would they know? You can tell when
φ becomes f when Romans start using that spelling. It's been suggested
that Spartan had þ for θ because some words with θ get spelled with σ.
But why did they think δ was a fricative in Macedon?