Every article I can find describes the 4.3¢/L as the estimated increase in price due to the introduction of the cap and trade system, and not a direct levy on gas.

https://www.fin.gov.on.ca/en/tax/gt/

Note: With the introduction of the Ontario Cap and Trade program, a regulatory charge has been added to the price of fuel and gasoline purchases in the province. The Cap and Trade program is administered by the Ministry of the Environment and Climate Change.

The 4.3c/L is a static charge that was introduced January 1, 2017. It remains the same today. If it were a result of Carbon Vouchers, it wouldn't continuously be 4.3c/L because Vouchers are sold on the open market and fluctuate.

A "regulatory charge" is still a tax.