“The bombs were reportedly triggered with model car remote controllers. Those have a range of hundreds of feet and do not require line of sight. They are radio-based, not optical.”
But we are talking about a big city street. A corridor of buildings. Plus the receiver/transmitter is on the ground. And they likely used a very low cost radio system. I think the useful range in this case is probably short and line of sight. It wouldn’t be, if used in an open field, though.
The hobby-grade R/C helicopters (from which I believe the receiver originated) typically are bundled with low power radios (under 10 Mw), but a higher power transmitter of 100 Mw or even more can easily be substituted. Range is often quoted at 2.7 miles for the 100 Mw (real, usable range in the clear), so a few city blocks are going to be no problem.
Today's dirt cheap radios are far superior to yesteryear's hi-tech expensive government-issue radios. A lot of work has gone into making them small, cheap, and reliable. They use advanced forms of modulation that require computer technology to encode and decode.
One issue that has been mentioned on other threads is the fact that a pressure cooker is a Faraday cage. Now that I think about it, I bet they placed the remote control electronics outside the cooker but inside the back-pack and ran only one wire through a small hole drilled in the cooker (the cooker could serve as the other wire). The actual detonator was probably a carefully busted Christmas tree light, as recommended by the AQ Chef.
The bombs were placed a block apart and within line of sight of each other. There were no tall buildings between the bombs, only people.