I have a magnificent St. Johns Wort shrub - Hypericum kalmianum ‘Gemo’. Multiple small St. Johns Wort plants are now growing in the surrounding area, apparently by self propagation of the seeds. The foliage looks the same but they have yet to flower. Are these "offspring" capable of producing flowers similar to the parent plant?

I don't see why not. They just may need to grow a certain size before they have the energy to do so.

Usually, and it depends on the shrub, seedlings could take several years to reach flowering maturity.