I hate that wording.
On or before closing means that it can be done any time before the buyer’s lawyer hits register. Sometimes, the seller actually takes that whole time getting the work done, and then the buyer has no chance to confirm that it was actually done, and done to an appropriate standard, before closing, which puts the buyer in a much more precarious position.
If you are looking to have work done by the seller before closing, you should require it to be done on or before the title search date. That should give you at least a bit of time before the closing date to make sure it was done, and get financially compensated if it wasn’t.