When there is a dam already producing electricity, you just reduce the flow through the dam rather than pumping the water back uphill. That is by far the most efficient way to handle excess generation from other sources in this case. This is a politically driven proposal whose implementation will be a boon to private producers at the expense of taxpayers.

This could certainly be a boondoggle, but reducing flow through a dam is limited to storing excess power at the natural flow rate of the river, whereas there is no limit in principle to how fast you could pump water from a big reservoir below a dam to a big reservoir above the dam, or how fast you could draw energy from greatly increased flow through your generators. So pumped hydro storage isn't an inherently silly idea.

A hydro turbine on a dam with no penstocks can probably go from 0-100% in 5 to 10 seconds.

The limit is pressure waves in the water conveyance system due to changes in flow through the turbine. If the water conveyance system is only the width of the dam and encased in concrete it is going to have minimal transients compared to a 7km long steel or HDPE penstock on which the time from 0-100% is more like 2 minutes