Manualspeak is sometimes so lovely 😉
This is about clock doubling (DX2 / 2x multiplier).
Stock 486 bus speeds are 16, 20 (these two 486SX only), 25, 33, 40 and 50MHz, and the original 486 ran at the same speed as its bus. When CPU (and so bus) clocks reached 50MHz, there were two general conclusions:
1) Still more speed was going to be required.
2) at 40 but particularly 50MHz bus speed, things started getting unstable. VLB cards and to lesser degree cache and system RAM had trouble keeping up, giving 50MHz bus systems a reputation for instability.
So, how to get more CPU speed without killing everything on the board (or upgrading it to then unheard-of speeds and prices)? Make the CPU run at a multiple of the bus speed. Thus the clock multiplier and the DX/2 (and later 4 and 5) was born. The CPU ran at twice the speed of the bus, so a 486DX2-50 had a 50MHz internal clock, but a 25MHz external (bus) clock, and a 486DX2-66 had a 66MHz internal clock and a 33MHz external bus clock. That's what that manual is referencing.
Of course, running the CPU at twice the bus speed meant that suddenly a lot of operations started to bottleneck on the now-slower bus clock, so a 486DX2-50 was always going to be slower than a 486DX-50, but in practice most tasks were so heavily dependent on raw CPU (ALU) performance that the hit from running the bus at half speed was only a few percent. A 486DX2-50 might have been slower than the 486DX-50 but the 486DX2-66 was faster in almost all applications. And both were far more stable than the 50MHz-bus monster. So the multiplier was a success and pretty much every CPU these days has a very significant multiplier. The performance costs have been mitigated primarily by fast caches at full CPU speed (L1 on the 486 and Pentium, later L2 and L3 as well) and later integrating the memory controller onto the CPU so RAM could be accessed faster than the system bus would have allowed.
So yes, your DX2-66 would be exactly the "66MHz (internal)" CPU the manual is referring to, and to run it at spec the motherboard needs to be set to 33MHz.