Since ten bikes had been built using this particular wrench, it works as a general purpose BBSXX tool. There are also beefier sockets on the market, and for an obstinate subject, I'd definitely use the socket
Start with a good penetrating fluid. If you are in a pinch, I've also seen acetone and ATF 50/50% mix used Acetone is fingernail polish remover, ATF is automatic transmission fluid. Let the threads soak for a while.
I've broken bolts and studs in half that were rusted solid (back in the 1980's), by simply adding a long pipe to a stout wrench to get more leverage (Cheater bar). When I showed the senior mechanic, he soaked the rest of them and used an impact wrench, which worked well. There is something magical that I can't quite explain about the rapid slapping motion of an impact driver. Its the air-gun at tire shops that makes a machine-gun sound, rat-tat-tat-tat-tat. (here is a one-minute video)
https://youtu.be/EX-EmWAr-_w?t=30
It provides a dozen light taps per second instead of one big punch. Was the frame steel? was it aluminum? were the threads cross-threaded? Was there galling? Did the installer use locktite fluid?
If the inserted part is steel, and the shell is aluminum, a technique that is very helpful is to heat up the shell. Maybe not with a torch, but an electric heat gun (used for heat shrink wire insulation) couldn't hurt the heat-treat of the metal (do not anneal heat-treated aluminum). Aluminum expands more than steel, so warming the aluminum shell just before you wrench on the inserted part can be very helpful.
The dogbone wrench shown above is handy (flatten it, and you could still use it), but if you are building ten ebikes, I recommend you also buy the socket and use a 1/2-inch drive impact gun for removals. Soak with penetrant fluid, and always start on the lowest setting and raise it until the locking-ring comes loose.