Many people with uninvolved biological fathers would disagree with you that the guy who impregnated their mother counted as their father, especially if they were raised by another man who actually did stick around, and that's even for dads who actually impregnated the mother. Sperm donation takes this even further because he claims to have actually done it anonymously [0], meaning he's as uninvolved as he possibly could have been in the process of being a father.
Many or most of these kids have real men who were actually there helping to raise them through their childhood who they refer to as "father", and it's pretty disrespectful of Durov towards those men to attempt to usurp that title on the grounds of what was supposed to be an anonymous donation.
I'll grant that Durov is more likely than most sperm donors to have some of these kids actually claim him as their father, but that's in no small part because there's now a substantial amount of money tied to them identifying him as such. Cynically I wonder if that's a major motivator for him doing this, because he knows that the kids wouldn't otherwise know or care who he is.
Keep in mind that he's inserting himself into their lives to give them millions of dollars.
Between these two choices:
1) Durov doesn't say anything, insert himself anywhere, or call you anything.
2) Durov gives you millions of dollars, but calls you his child.
I bet my life savings that every single kid will be fine with being called Durov's child in exchange for millions of dollars.
I agree that it's pretty cringe to refer to all of them as his children when he's literally a sperm donor, but he definitely did call them that.