Screws into 1/2" ply should be fine. It only has to keep it from shifting around on the table, correct?
I think there are a lot of frankly over-engineered designs on here. I built a Paulk Workbench (one of the original torsion box workbenches) a number of years ago from 1/2" ply and pocket screws. It’s held up through insane conditions, a cross-country move, and has been hacked into 3 different configurations and it’s still going strong as a miter and router table station. The Paulk plans only have 1/2" ribs every 24" where some here have 4x that amount. Paulk ribs are hollowed out to allow access as shelves, most of these are solid. I wouldn’t sweat screwing the OF into 1/2" ply just to keep it stationary on the table.
