They could have used a lower-clocked version of the one in the 2018 MBA (1.6 GHz, i5, 7 W TDP, 8270Y, Amber Lake), eg, the 8200Y (1.3 GHz, 5 W TDP, UHD 615 graphics).Intel stopped updating the CPU used in the 12” MacBook. Apple has nothing to update it to.
The 2017 MB came with the 7Y32, Kaby Lake, 1.2 (1.1?) GHz, 4.5 W TDP, HD 615 graphics.