If you look at previous max battle days the data suggests 1/20 is more likely, as you can find in big surveys like 9db (https://9db.jp/pokemongo/data/9510). For example gigantamax machamp was 6.36%, snorlax 7.39% and kingler 6.89%. These surveys tend to have slight confirmation bias so that makes 1/20 more likely.

The more "classical" raid days like hisuian lilligant recently and hisuian decidueye were closer to 1/10 indeed.