I wouldn't disagree with you about on-line training working for HF Refresher and Fuel Tank Safety Phase 1 training, although it misses out on the interaction that occurs in a group of professionals in a well-facilitated HF training session.
But my point was that the Certificate that follows simply cannot stand up to a robust audit, unless the on-line course was carried out under supervision. By definition, it is impossible to show beyond doubt that the holder actually carried out the course and/or a test after it if there is one, unless someone accepted as trustworthy makes a legally binding declaration to that effect by endorsing the Certificate with an auditable signature.
One day, somewhere in the future, there will be a way that exams can be taken online with the same security as invigilated exams under Part 147 conditions. But that's a long way off. The same applies to mandatory training such as HF.