Your first assumption is false. Radar/lidar has been capable of compensating for being in a moving vehicle for over a decade (more like 2 decades now). For you leadfoots out there, there are a lot of cruisers and even motorcycle with both front and rear facing radar now.
Depending on the technology, it may or may not be very accurate pinging a bicycle.
Usually if it's a speeding ticket, your best defense is that cycles aren't >required< to have a speedometer, so there's no legally obligated reason for you to know how fast you were going, regardless of whether you actually have a cycling computer or not.
Disclaimer: I'm not a lawyer, and I don't play one on TV.