Essentially, yeah? Unless you calculate the OTPs by pen and paper, you have to use some kind of software, and therefore you have to trust that it is safe. Writing your own like OP is actually a very safe option, because you can trust yourself, but everyone else needs to trust OP.

Attack vectors apply to the add-on itself, it is (potentially) the shady site. OP has the potential to update the add-on later with its own malicious code. This is true of all addons, hence the trust issue.

I dont have any problem with OP advertising their addon, but potential users should be aware of the risks.