Grid forming typically refers to inverters connected to a large electricity network. What you're talking about is islanding, ie running a system separate to the grid when it would normally be grid following. The principles are similar, in that both involve using internal voltage measurements to control the generation output (rather than externally chasing the grid voltage), but the practical nature is different - grid forming systems have to deal with large fluctuations from the network, well beyond what you would see in a domestic system. The terminologies overlap a lot, but grid forming specifically refers to large scale systems and more complicated networks.