Is this trap arm soldered?

1960s-era plumbing (galvanized pipe in wall - I know it needs changing and plan to do it next year when I remodel this bathroom, but not ready to do it yet.)

The trap and trap arm are thick-gauge brass and the bottom of the trap finally corroded away. I am replacing the trap and trap arm all the way to the wall. When I removed the escutcheon plate, I was surprised to see what looks like a bead of solder all around the trap arm where it fits into whatever is in the wall (looks like brass.)

Am I correct? Why would that be a soldered connection? I am afraid to just twist the trap arm to break the solder and hopefully unthread or just pull it straight out, mainly because if things go wrong and the tube gets mangled then I'm busting into the wall and getting into things I am not planning to do until much later.

A last resort is I can cut the trap arm a couple inches from the wall and install a compression coupler to transition to the new trap arm, which from experience I know will work.

All ideas welcome!