It's basic, philosophical Hinduism, thousands of years old; also grokked by non-Hindu mystics throughout history.
The multiplicity of 'things' and beings, in our subjective, material world, is also illusory.
Only a single, timeless, Unified Consciousness exists. All these lives-after-lives, of multifarious, individual consciousnesses, is no more than a subjective dream, emanating from a single source, like multiple pinpricks of light from a colander inverted over a light bulb.
We create/dream our own realities; our own lives, over and over -- till we wake up and merge with the light bulb.