Let's see... Heavily armored, strike from the skies, meant to be larger-than-life heroes, created by a God-Emperor to reunite the kingdoms and drive back the savages...
Yes, they're fantasy Space Marines. Just like Necrons are Space Undead, and Eldar are Space Elves. They stopped short of giving them rifles because their ranged weapon naming convention would have meant they'd be using "boltguns." They're AoS's version of Space Marines, and trying to deny that is just silly, no matter how much you feel you have to rush to GW's aid. They even replaced their statue of a Space Marine with a statue of an Eternal.
Of course there's differences... just like there are differences between Undead and Necrons, but we all still know the Necrons are Space Undead. Only people so in love with GW they'll deny the obvious would claim there's no correlation between Space Marines and Stormcast Eternals. I doubt that the design studio would deny it if you could get them "off the record." I don't know why people act like it's something dirty, either.