And this is why Manufacturers shouldn't be allowed to self certify
This doesn't sound like a certification issue so much as a process compliance one. Someone skipped a manufacturing step involving the fuel system which resulted in a leak. And subsequent QA inspections didn't catch it. It's quite likely that the FAA reviewed Boeing's manufacturing and QA processes and signed off on them at one time. And they may have performed subsequent spot inspections. But QA is something that manufacturers do on their own.
The CBC article doesn't make it clear whether Boeing's self disclosure was made independently of the leak being discovered or as a result of it. Either way, right now might be a good time to have scrutiny increased on the shop floor. 737 MAX production is taking a hit right now and it might be better to take this opportunity to slow everything down and find out what else may be slipping through the cracks.