Unless I am heavily misreading, “we organized a strike, and if you don’t join us, you are expelled from the union” sounds exactly like a union forcing its members to strike.
What other kind of “forced to strike” scenario were you imagining? Union leaders holding those refusing to strike at a gunpoint? Because other than extreme examples like this one, it sounds like a textbook version of forcing members to strike.
Most if not all organisations have standards of conduct that they expect their members to follow, and its not surprising that unions view strikebreaking ("scabbing") as being contrary to those standards.
But seeing as the whole point of unions is organised action, why would you want to be a member if you weren't interested in participating in that action?
Employers on their part have access to a similar tool in the form of lockout, where they bar employees from coming into work and stop paying their wages. An employer targeted by (legal) solidarity action can't fire their striking employees, but could respond with a lockout.
There's a provision that disallows strike action where it would harm public order, which would apply to the police union refusing to protect Tesla property.