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.

I've been a consultant for a long time and in that role you were hired only to fix old systems that the resident staff refused to maintain, because not sexy. I didn't mind, it paid good money and I have seen a lot of companies from the inside which is also a plus. The dev were not below average, they just weren't interested.