What Kinguin sells is someone sending you a Steam gift of the expansion.
Which is not allowed according to the Steam TOS, both selling and buying Steam gifts that way can lead to the permanent suspension of your account.
So i would not risk it.
It would work since its the Steam version of the expansion but again, not allowed by Steam.
Also looking at the price Kinguin wants for that "gift" you are much better off waiting for the next sale on Steam.

Originally posted by groover:

Hello,
I have a Steam account linked to Battlenet and I own Diablo IV on my Steam account.
I missed the Steam sale by a day and found Vessel on Kinguin for a reasonable price.
The site is offering me Diablo IV: Vessel of Hatred - Standard Edition PC Battle.net Altergift, but I don't know if this version will be recognized by my Steam account.
If anyone has any tips, please help me avoid making a purchase that won't do me any good.
Thanks.

If it says battle.net it sounds like its for battle.net, not steam.