They do it two stupid ways.
One is the one year you referenced where they borrowed (took out loan or bond) to pay for their stadium renovations. They claimed this $400M as Revenue in their aggy math P&L, and so claimed to the world that they were the richest based on this. Of course this has nothing to do with revenue and is a balance sheet addition for cash/loan in and an outstanding obligation for the same. But of course...aggy.
Second is where they claim 100% of the PUF as aggy endowment (not the 1/3 that is flowed to A&M system), usually closely followed by saying that makes them richer than Texas who only reports 2/3 of the PUF as endowment as that is what goes to the UT system. So their logic has them claim 100% of what should be 1/3, and when Texas rightly claims 2/3 they brag about how much richer they are than Texas.
Of course ignoring this if we look at aggy endowment outside of the PUF is around $2.5B and Texas' is $13B.
But aggy richer and biggest and bestest of everyone.