Not sure what recipe that is, but usually there is no added oils. These can get rancid even in home canning. There is also the issue of adding citric acid or lemon juice to ensure that you have sufficient acid. For 10 pounds of tomatoes that are not a meaty plum type, it could have been cooked down quite a lot to get to your sauce thickness preference. Some people prefer a thick sauce to a thin one. I prefer thick, almost to a ketchup point. Since I like thick, I usually use San Marzano and other plum types that are bred for sauces. For spices, oregano is ok to can, but basil seems to get lost in a sauce unless you use a few whole springs/leaves. A few times I like to add dill instead of the traditional seasonings. I think that amounbt of olive oil may be too much for just 2.5 pints. Oil does float and that amount is going to definately show up on the surface once canned. Refrigrate the product and use within two weeks if your concerned about the oil amount. Here, when I can tomatoes for sauce I usually make batches of 10+ quarts at a time, and use no oils of any kind. In some of my canned tomatoes, I don't even add salt, for the healthy people.

Forgot to mention, I processed them in a BWB. The recipe is from the Blue Book, so I'm sure it's reliable, even though it includes oil, and it's probably widely used.

The tomatoes were not plums, but the recipe doesn't specify that they should be. Ten pounds of plums would definitely produce more sauce of a given thickness than 10 lbs of slicing tomatoes. Most recipes that I've seen don't specify tomato type or caution about a recipe not yielding enough, so I'm beginning to think it probably isn't an issue for some reason.

You omitted the onions, so you don't have as much sauce, is my guess. The onions are low acid, so I think that wouldn't hurt anything, it is fine to omit low acid veggies. I try to just sweat out the vegetables in a pan without adding the oil when possible.
It should be fine.
If it concerns you, then you could pop the seals and freeze the jars of sauce.

Yes, I guess the onions would make up some of the difference. Three sauteed onions...maybe a half pint. I suppose it's safe to assume that they understand that the yield will vary depending on tomatoes used and desired thickness. And have planned for the worst case scenario.