Controlled impedance routing is enabled when there are plane layers in the stackup.
Altium's impedance control is very basic, limited to setting the trace width during routing, based on the calculated value. There is no check for adjacent copper: a very large polygon pour will not be considered a reference plane (because a polygon is routing copper), and conversely, you are not prevented from routing controlled-impedance traces over a plane keep-out region.
If you need accurate trace impedance, signal quality, or SWR calculations, consider getting an RF-oriented board simulator.
Tim