FFS! Incumbency is a complete non-factor in corruption. It's always the ability to increase your power, while in office.

Three countries with fairly open and competitive elections, and very high levels of corruption and high levels of incumbency:

https://www.transparency.org/en/countries/india

https://www.transparency.org/en/countries/turkey

https://www.transparency.org/en/countries/hungary

Then there are these least corrupt countries, that elect new people all the time:

https://www.transparency.org/en/countries/finland

https://www.transparency.org/en/countries/new-zealand

Oh... and let's not forget, that the study is about how efficient the governors are... not about corruption at all.