meaning - What does the author actually means when he says " underestimate the value of gold."

Let's first look at what Jeffery actually meant to say by looking at the context.

"This is a really sensitive issue we don't want many people to know about, so "silence is golden" would be an understatement in this case."

"silence is golden" means that being silent is a good and valuable thing.

To underestimate the value of something is to think it's worth less than it actually is, so in this case, we're diminishing the value of gold by suggesting silence is golden.

The problem I have with this is that if you turn this around he's saying that gold is more valuable than silence is, which is the opposite of what he meant.

I could be completely wrong on this, and feel free to downvote my answer into oblivion if you disagree.