I take his statement to mean the average annual revenue has increased 21% in total, not that it has increased 21% annually.
Braves end of year payrolls since 2016 (the season before SunTrust) have been (roughly):
https://legacy.baseballprospectus.com/compensation/cots/national-league/atlanta-braves-2/
2019 $144,000,000 (from memory that I read somewhere)
2018 $136,024,060 (19th in MLB)
2017 $133,400,668 (20th in MLB)
2016 $128,348,481 (17th in MLB)
An increase of ~21% over $128M is $154M, and exactly in line with what we expect the 2020 year end payroll to be. Keeping $10M-$15M in reserve sets the opening day payroll around $140M...which is still awesome and give them almost $40M left to spend (I have the current payroll at ~$102M).
He could have meant 21% per year, in which case they haven't been pumping nearly that much back into the roster.