That’s an interesting paper. If I’m reading it correctly then a lot of the water was pushed up into the stratosphere and is above where our weather systems form? The water that did get added to the troposphere would surely have been lost by now?
The Tonga Volcano article reports up to 146*10^12 (Teragrams) of water being ejected into the Stratosphere
The Greek floods the other day show (by my reckoning on the maps published) an average of 200mm of rain falling over an area of about 100km x 100km with some areas inside this square totalling about 700mm but mostly in the range 100-200mm
Using my trusty fag packet the Tonga volcano ejection comes to 0.146Gt (10^9 tons) of water whilst just the Greek flood total on its own comes to around 2 Gt rain
My conclusion is on that basis the Tonga Volcano was not much if any of a contribution to the recent floods