I'd have to read the piece. A few splashes of cold water:
More than half of the surplus over the decade is from payroll taxes. These would be expected to hold up better than income taxes. Even if income tax revenues tanked sufficiently to eliminate the 'on-budget' surplus, there could still be an overall surplus. Payroll is projected taking into account the likelihood of recessions.
Don't forget capital gains enjoy preferred rates, so the revenue proceeds are not as high, hence the reduction of gains occasions less of a revenue loss. Capital loss deductions from other income (i.e., in excess of gains) cannot exceed $3K in a year (the rest is carried over, hence the revenue loss is spread out).
I've pointed out the partial and temporary effect of capital gains on revenues in the past. Taxes do derive from dividends, interest, and profits. Much of the buzz about market valuations stems from their outpacing mundane these literal revenue streams. It could follow that slower growth of valuations is similarly absent from the picture of literal revenues. To this extent, a market drop would not reduce tax revenues from capital income.
mbs