Pools put a recognisable string in the coinbase transaction, and explorers map payout addresses to pool names. Counting blocks by label gives a picture of who is coordinating block templates.
It does not give ownership or control of the underlying machines, which belong to thousands of miners who can switch pools, nor does it reveal whether two labels share an operator. The Gazette reports pool shares as dated observations from a stated window and says so each time.
Concentration of labels is worth watching; it is not the same as concentration of hashrate.