Maybe "COBOL" here is a misleading shorthand for mainframe banking programming?

The original quote doesn't say anything about COBOL, or about mainframes for that matter. It speaks about "systems only one or two people understand", which seems normal enough. You can write incomprehensible software in any language if the specs are convoluted enough, and a bank that has existed for over half a century will almost inevitably be a mishmash of poorly integrated systems written over multiple decades.

Sometimes at different banks which were subsequently merged with, sometimes by a consultancy which has long since gone bankrupt, sometimes with a compiler that is no longer available. It's a miracle the system works at all.