So I've been wondering for a while if the development of the Neptune as a cruise missile, rather than an anti ship missile is a bit of a work around for everyone. The Ukrainians obviously have the knowledge on how to build one, but the secret sauce is the guidance. I know these missiles use either internal navigation or satellite navigation to fly to their targets. In lieu of giving the Ukrainians cruise missiles from the US (and approaching another bullshit red line), it would have to be easier to covertly give them either guidance data or access to nav satellites ("hey guys, if you happen to just modulate to this code at this time, the satellite will just happen to grant you access, wink, wink").
I'm not trying to take away anything from the Ukrainians, as this all may well be wrong. Either way, whenever this is all over, there will certainly be an interesting story to tell on the development of this. Of course it will be years and years before it is ever told, but eventually it will be.
In the short term, because this is a Ukrainian weapons platform, it gives them more stocks to hit within Russia, I hope they can build a lot of them quickly.