Very interesting! I haven't seen the film (or any Argonauts film version); I've been meaning, though, for about two years to read full translations of the various Argonautikai, since I enjoyed J.S.-K.'s Geosophia so much, have read a lot about Medea over the years (as well as Hylas), and can see many potential parallels with a number of things--including the Irish immrama--with the tales in general.
Also: the "unique Deity" nature of what you discuss relating to those Goddesses is very interesting as well…Irish has a way of just using the definite article (there is no indefinite article) to do the same where certain names are concerned: An Dagda, An Morrígan, etc.; but it can also happen more generally with heroes and figures of renown as well, so that it can imply "The (famous) Cú Chulainn" and so forth, where a name might be more common but in a particular context is being used to indicate (as we'd say in English) THE Conchobor, Cormac, etc.
Or, to be almost disrespectfully colloquial and modern: it would indicate "The (Goddess) Brigit" (or one of them, anyway!) as opposed to "this basic bitch Brigit." ;)
Wow, I'm surprised you never came across the film on television. I first saw it when I was quite small and it made a very deep impression on me, not only in the sense of sparking a lifelong fascination with the Argonautica, but also for modeling polytheist piety in some memorable ways. It's a rather pious film, amidst all the adventuring.
It's a Harryhausen film, yes? I saw the original Clash of the Titans, which (despite several problematic aspects) had a profound influence and impacted me deeply, but never this one…I don't recall it ever having been on TV that was accessible to me, to my knowledge; if it was on some random Saturday afternoon in the 80s, I'd not have been allowed to watch it, alas. :(