If the NSA has broken them, they probably aren't the only ones. By keeping that a secret they're actually endangering national security.

They won't show what they have access to and actively use against adversaries. They have "NSA approved cryptos" such as TACLANES hardware that other countries can use at government level with their own crypto bases. Civilian level encryption was never meant to be Government level, we were only lucky that AES and such was opened up by US law in the 1990s.

I'm having trouble seeing how that makes any sense.

When the Allies in WW2 broke Germany's Enigma cipher, are you suggesting that by not telling Germany that the Allies could now read a large fraction of Germany's military communication they were endangering their own national security?!

That makes no sense whatsoever.

If the NSA has broken whatever cipher suite the GRU uses, or the Iranian RG, or whomever, please explain how keeping that fact a secret endangers the USA?

Keep in mind they share info with the other Five Eyes and select other allied nations.