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

Excerpt from Honor Among Thieves:

"The reason for numbers being limited on this occasion will become all too obvious to you. The issue we are about to discuss is so sensitive that the fewer people who are aware of it, the better. To suggest in this instance that silence is golden would be to underestimate the value of gold."

Here what does Jeffery means when he says "underestimate the value of gold"? Does it means gold is really precious and at this point silence is something that they couldn't afford and if they do that it will 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.

Silence is golden:

One ounce of gold = one unit of silence

But the above underestimates the value of gold.

One ounce of gold > one unit of silence

Literally he's saying that silence is less valuable than gold. This clearly is not what was intended, but it's not at all unusual for someone to get their conceptualizationizing all bolluxed up when trying to stretch such an idiom.

"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 not so valuable as gold, it is even more valuable