What do you base this on?
Personal experience. I have been soldering stuff for almost 45 years. That's why you need flux, remember?
I have some cheap solder that is oxidized so bad the oxide comes off in your hand, and the solder still on the roll!
Solder has an oxide that is a white powder. CU's are either a red or a black powder.
Just by your own observation, how many times have you seen a solder joint covered with white powder vs. bare copper covered with red or black powder?
Also, copper forms a patina that is electrically conductive and seals the rest of the metal from water and air, just like bronze does.
Solder has it's uses and I am a big fan of soldered connections vs. crimped, but have seen many failures of tinned stranded wire under pressure connections.
Solder should be totally independent of the mechanical component of a connection. Ideally, the solder should not be applied until after the mechanical connection is made.
If you work on electronics at all, you will know that solder is used on PC boards for mechanical connections. If you have done any repair in the electronics field, you will know that the #1 failure of a board is a 'cold solder joint', or a 'dry solder connection', both of which would not exist if the solder wasn't used as the mechanical part of the connection. If you have worked with bad solder connections and use a microscope (like I do) to locate them, take note that the ring formed by the separation of the joint on a flat board is covered with oxidation nearly 100 percent of the time.