Something like that, yes. Some providers bind the SIM to the phone. Some work with partner carriers so that you can use the partner’s SIM but still cannot move freely to any other carrier. Some do not encourage the traditional behavior of moving SIMs from phone to phone. And then there is the issue of not being able to put an old SIM back into the phone that used it after putting a different company’s SIM in the phone.
At this point I think the most efficient course of action would be have Boost sort it out, especially since that course of action works for you as well.