I'm a full-stack software developer working in the financial sector, their statement is factual.

Companies will never want to take on liability that has the potential to bankrupt them. It is in their best interest to not reveal the version of libraries they are using as some versions may have publicly known vulnerabilities, and it would make it incredibly easy for attackers to build an exploit chain if they knew the exact versions being used.

Securing client code is just as important as securing server code, as you don't want to expose your users to potential XSS attacks that could affect the way the page gets displayed, or worse, leak their credentials to a third party. If this happened in the EU or some parts of Canada, and it's been found that the company reduced their threat model "for the sake of openness", they would likely be fined into bankruptcy or forced to leave the market.

Unfortunately, this is one of those cases where your interests and ethics will never be aligned with those of service owners as they are held to a certain standard by privacy laws and other regulations.