Yes, they are the categories or masses (kāyā/khandhā) which make up reality, which exist (sat-). The metaphysical notion (diṭṭhi) regarding them is of a self being equal to, in possession of, outside of or inside of them. As I wrote in my post on Sakkāya, I think this is a dialectic like the Buddha’s “definition” of ‘sabba’ is. It is not sheerly literal; it is conversational.
‘Sarva’ in the Upaniṣads is a problematic term from the Buddhist POV. But the Buddha used and recycled it in a dialectic way. What people are referring to in their theories of the All or Whole is actually the sensory domains. What people are referring to in their theories of substantial categories are actually five insubstantial aggregates.
That is one way of taking it.