The said refactoring was probably not a huge task since he is reporting it done. Larger tasks are most often planned and monitored.
Splitting some class and decoupling some resources + adding the testing framework and implementing a first batch of tests. Of course it might have introduced few regressions.
But
> has potentially some known issues
Read again that. Exactly, none knows how many bugs there are, only how many were found so far. Bug tracking is often lacking, behind schedule, wont fix, etc.
On the contrary modern code analyse tools are frightening good at spotting untested conditions every where (sonarcube comes to my mind). Much better and much reliable than human developer and human tester. Sonarcube does not care if the case can not happen because user interface does not allow the input. The bug is here, and the bug is spotted.
The more experienced person is plain wrong : more tests is better. It's not panacea yet, but it's better.