I really think "delicious" is not the best translation for "νόστιμος". "Delicious" is a bit of a superlative (νοστιμότατος, πεντανόστιμος κλπ) while simply "tasty" is a far better match.
cows' milk is a "typo"? So this cow's milk is tasty but some other cow's milk might not be tasty?
Yes - because the sentence has της αγελάδας (genitive singular), not των αγελάδων (genitive plural).
Would you write the same for the cow's milk is tasty and Cows milk is tasty
The second one would be incorrect unfortunately, unless you meant "Cow's milk is tasty". In that case, that second version actually sounds better.
In greek when we say το (γάλα) της αγελάδας είναι νόστιμο we mean generally. If it was for a specific cow we would say το γαλα ΑΥΤΉΣ της αγελάδας.
Well, from that perspective "Το γάλα αγελάδας/Το αγελαδινό γάλα..." would be equally correct for a general approach. However, the version with the article is not incorrect. One could be talking about a specific cow, without using αυτή/εκείνη.
Το τρίχωμα της γάτας είναι μαύρο. - The cat's fur is black.
Ο σκύλος έχει μικρά αυτιά - The dog has small ears.
The article is there, yet not all cats have back fur and not all dogs have small (short) ears. These cases are not necessarily general, so this one might not be either. It could be, but it's not something too obvious, so it doesn't need to be.
So, since there is that ambiguity, both versions (with and without the article) are included in the English translations. ^.^