I dont think thats correct... if you look at FCC STD-019D it specifically states that soldering can be used except in the ground and lightning supression systems (which would make sense since a lightning strike would vaporize the solder). Other than that there are even some parts of the spec where soldering is recommended (like for high frequency/shield connections).
There are some stipulations like that the connection you solder to should have mechanical support for the wire (e.g. the solder does not provide the mechanical support) but otherwise its showing to be ok...
Yes, antenna cable connectors and such are soldered, but if a standard wire is damaged and spliced both of the A&P mechanics I use have always used approved connectors. Neither has ever had a soldering iron in sight and only a connector kit with a ratcheting crimp tool. I actually mentioned using solder once and he said it was not approved. Unfortunately, my mechanic has to sign off the maintenance logs when any work is done on the plane so without his signature, the work would be deemed not approved.