The T9 bus service is part of the Welsh Government's TrawsCymru regional bus network, though I believe Cardiff Bus are contracted to operate that service for them. (All TrawsCymru services are numbered with a 'T' prefix.) Funding and ticketing is separate to Cardiff Bus' other operations, though.
From July 2017, weekend travel on TrawsCymru routes was made free of charge, including the T9 route between Cardiff and the airport. The scheme was originally due to run until May 2018, but was later extended. (It is currently suspended due to Covid issues.)
The 23rd of August 2019 was a Friday, so the service shouldn't have been offered free of charge on that day (but would have been on the 24th). Perhaps you were on an overnight flight that arrived on the morning of the 24th? I wouldn't expect that the driver would be offering free travel on other days of the week, as the Welsh Government was only reimbursing operators for free travel on the weekend.
The friendly, jocular nature of the bus driver's explanation is typical of the Cardiff area, where passengers are noted for saying "Cheers, Drive" as they disembark.