IIUC, in the future, the SS tax isn't going to cover the benefits that SS will be obliged to pay out. So some analyses model the shortfall as coming out of the government's general budget.
So, yeah, it's "separate" spending in a sense, but it's not totally in its own sandbox.
This is already the case. Social Security has been running a deficit since 2010. It's only stayed solvent because of the trust fund, which is expected to run out in around seven years. So the government is currently keeping it afloat using general budget revenue.
So, yeah, it's "separate" spending in a sense, but it's not totally in its own sandbox.