
I've always thought of this as a description of where Saruman got his Orcs, specifically the stock he used to breed his Uruk-hai: he used uruks from the Misty Mountains. Notice, these uruks are described as being both from the Misty Mountains and in the service of Saruman. Saruman's breeding programs were only just getting started between 2991-2995:From Appendix A, the House of Eorl
In 2989 Théodwyn married Éomund of Eastfold, the chief Marshal of the Mark. Her son Éomer was born in 2991, and her daughter Éowyn in 2995. At that time Sauron had arisen again, and the shadow of Mordor reached out to Rohan. Orcs began to raid in the eastern regions and slay or steal horses. Others also came down from the Misty Mountains, many being great uruks in the service of Saruman, though it was long before that was suspected.
These Orcs would have considered themselves Orcs of Sauron, as they were in origin 'his creatures':From Unfinished Tales, the Palantirí
The Council seems to have been unaware, since for many years Isengard had been closely guarded, of what went on within its Ring. The use, and possibly special breeding, of Orcs was kept secret, and cannot have begun much before 2990 at earliest. The Orc-troops seem never to have been used beyond the territory of Isengard before the attack on Rohan. Had the Council known of this they would, of course, at once have realized that Saruman had become evil.
From Appendix A, The Stewards
In the last years of Denethor I the race of uruks, black orcs of great strength, first appeared out of Mordor, and in 2475 they swept across Ithilien and took Osgiliath.
From Appendix B, The Tale of Years
2475 Attack on Gondor renewed. Osgiliath finally ruined, and its stone-bridge broken.
c. 2480 Orcs begin to make secret strongholds in the Misty Mountains so as to bar all the passes into Eriador. Sauron begins to people Moria with his creatures.