It could work either way. It would be better to phrase it as "outside of turbo mode it operated as a 4mhz 8086, with turbo it operated faster (usually the CPU's actual rated speed)".
Hah that reminds me of the way of a supermarket chain to shill rebates.
Turbo button not pressed, run at lower speed than rated
Turbo button pressed, run at rated speed
No rebate, sell at a huge margin
Rebate, sell at a minimum margin
The linked article suggests it could be both ways for button behavior. But I see what you mean for how it achieves it.
Yup, on many boards you could either jumper the direction (turbo switch closed= slow or closed = fast), or set it in the BIOS.