First of all, to my knowledge, Tolkien certainly never "admitted" that orcs were a "racist" creation. I haven't read every word the guy ever wrote though, so if I'm wrong, I challenge anyone to provide proof. Otherwise, let's assume no such thing ever happened.
My understanding is that Tolkien acknowledged that he based the idea of certain aspects of orcs on the Mongols (among other groups): a tribal, largely nomadic and warlike people who were greatly feared in their time. He basically took certain elements of an historical "real world" culture of humans and applied them to a "make believe" fantasy culture of humanoids... In other words, he did what any author does when creating a fictional race of humanoids.
Now, if you want to claim that basing certain aspects of make-believe humanoid cultures on certain aspects of real historical human cultures is axiomatically "racist," fine-- but by that rationale, every single make-believe culture is inherently racist, because they are ALL at least partially based on the creator's understanding of human culture. Even the most bizarre, alien humanoid race in a SciFi world is going to be based in some way on author's understanding of human culture.
I think that's a very, very ridiculous concept of the idea of "racism," and one that would quickly lead to the elimination of any fictional work if taken to its natural conclusion. But I also think that most of those who buy into the whole "racist orcs" theory (at least the smart ones) would argue, "Hey, it's not just that Tolkien attributed elements of a real-world culture to a fantasy group of humanoids, it's that he drew a clear parallel between a real-world culture and an evil fantasy culture." The implication here being that orcs are simply a thinly-veiled disguise for the "Mongols of Middle Earth." And orcs, as we all know, are dim-witted, lazy, and irredeemably evil creatures.
This is where nuance comes in, so it get's a little tricky (because people on either side of any issue seem to struggle with nuance these days). Suppose Tolkien had said, "Sure, orcs and Mongols are the exact same thing." Well, at that point I'm afraid Mr. Tolkien is being quite the racist. Necessarily so, because orcs are (by his own definition) INHERENTLY evil, lazy, stupid creatures. The word "inherently" is super important here, because it's not just that orc culture tends to emphasize laziness, savagery, and stupidity, it's that orcs were intentionally created that way. Those pejoratives (lazy, savage, stupid), aren't just descriptions of typical orc culture, they are irrevocable attributes of the orc race itself. Taking a baby orc and putting it in a Skinner box to try and create a refined artist would never work (according to everything we know about Tolkien-lore), because a member of the orc "race" would be incapable of denying it's naturally barbaric and stupid nature to embrace refinement.
So, sure, if Tolkien believed the same thing about Mongols- that they were created by some deity to be a naturally and irredeemably savage, violent, barbaric race of people- and he created orcs based on those beliefs about Mongols, well, then I think you have a pretty good argument that orcs are a racist creation, because you'd have a pretty good argument that Tolkien himself was racist. But I have yet to see any evidence that Tolkien believed that about the Mongols or any other race of people. Taking certain recognizable aspects of an historical culture (any historical culture) and applying those aspects to a fantasy culture in and of itself isn't close to crossing the "racism" bridge in my book, no matter how evil the fantasy culture is.
So unless you can demonstrate to me that Tolkien believed those arguably negative qualities of the Mongol people to be inherent elements of the Mongol "race" rather than simply hallmarks of the Mongol "culture," well then I'm going to have to decline to buy into the whole "racist orc" theory.