99 is the highest you can stack some items, like wood or stone. Most other items, gems/food/flowers cap at 50 I believe. Seems most likely they just grinded for wood and stone by placing a lot of trees - fairly quick to do if you can stand having...
I don't have a modded switch, so I wouldn't be able to show this, but there are plenty of videos on youtube of folks using save editors/injectors with custom firmware on their switch to retrieve game files from the system, edit or add to them on...