It's pretty well known that it is flawed bullshit when applied a small number of times to a small number of people that you have limited information where you are trying to find out the truth about something very specific.
Applied repeatedly over a long time to a large group of people that you have extensive information on where you are aren't looking for information about some specific thing, by an agency with extensive experience in extracting meaningful signals from noisy sources...I would not be at all surprised if they can get something actually valid from it.
They don’t. Unless you count giving a Lebanese Hizballah illegal a security clearance, which might have been due to something other than the polygraph but is hard to reconcile with any background check actually being performed.
Polygraphs could work as advertised for 99.99% of people being processed for security clearance and still be useless, because their effectiveness against people who take the job with the intention of leaking, selling, or otherwise inappropriately handling classified information is nil. The US government has spent tons of money on polygraph research and administration over the last century despite knowing from the beginning that it was a fraud, and has never even once prevented an act of espionage with one.
Applied repeatedly over a long time to a large group of people that you have extensive information on where you are aren't looking for information about some specific thing, by an agency with extensive experience in extracting meaningful signals from noisy sources...I would not be at all surprised if they can get something actually valid from it.