Seems to me like it's a full hechsher on the items quoted. Aren't the soft serve items from his machines?
The cert is for the items that are packaged and sealed as those brands, and bear an OU-D on the sealed package. So yes, if he tries to sell you a sealed mix with the OU on the wrapper, It would be OK.
Think of it as if you go to a party, and there are cookies on the platter, and someone who does not keep kosher shows you the OU cert for oreos. True oreos are certified, but who says that these are oreos. and even if they are, are all the cookies on the platter oreos, and was there chazzer on the platter yesterday.
Rule is, once the package is opened, the OU does not apply, and you are relying on someone else.