No, they don't know your secret

It's just random extortion, as explained in the Washington Post:

“I know about the secret you are keeping from your wife and everyone else,” a mysterious letter read. “More importantly, I have evidence of what you’ve been hiding.”

The letterwriter, GreySquare15, threatened to expose a secret — unless Strohl wired the sender $15,750 in bitcoin...

Strohl, who lives in Washington’s Chevy Chase neighborhood, has been happily married for 14 years and said he recognized the letter was a likely scam. After posting about it on a community listserv the next morning, he realized he was one of several residents in the area to receive similar threats in the past month in a scheme the FBI says appears to target affluent neighborhoods across the country...

The customized blackmail threats using a name or other personalized detail to bolster the all-knowing tone in the letters may draw on names and addresses found on publicly available sites or have been acquired through private data the fraudster bought.

“Because of the amount of people’s personally identifiable data out there on the dark Web, criminals can purchase this type of information and attempt to use it against you,” Ames said. “But it’s a scam, and folks should not pay the demand money.”

The fact that someone appears to have access to passwords or other information — even if the information is out of date — can be unsettling all on its own.