A single person in San Fransisco has a cost of living of $1125.83 without rent, according to numbeo (whatever that is). Median rent in San Fransisco is $3700 for a one bedroom.
That leaves a measly $230,736 of post-tax, post-living expenses money. Assuming this person does nothing more than hide a monotonically increasing pile of cash under a mattress, it takes just over 43 years to amass $10,000,000.
Presumably this financial cretan started working right after college, at the age of 22. This means they've acquired the full sum at the age of 65, which is not an uncommon retirement age. Cuts close, but checks out.