
Simply uncheck the Asset Class checkbox which is checked by default, and assign the security to whatever asset class you want.
But I agree with Quicken Eli's explanation about why they programmed it the way they did: viewing Portfolio Values by Asset Allocation, you take advantage of driving into the asset mix of your holdings, whereas viewing by Performance, you don't because they can't determine the performance broken down by asset class. It sounds like you want it to do both: use the asset allocations in one place and ignore them and assign a single asset class in another. I don't think that would be easy to implement, nor easy for most Quicken users to understand.
Personally, I find with the Portfolio view filtered by Performance, I'm mainly interested in filtering by Security — I want to see the performance of each of my securities. (Same for gains.) I don[t feel I have much need to see the performance of each security broken down by asset class, especially if I know it isn't accurate data. If I want to see asset allocation, I switch to filter by Portfolio Value.
If I want to see how different types of investments perform, I can look up performance of various indexes. Now I do wish we could add selected indexes — which already exist in Quicken Mac but can't be seen anywhere except the list of securities — to things like the table and chart in the Portfolio view, so we could see how our securities fared performance-wise against various indexes.