I buy it. It is fairly well known that too much or the wrong kind of debt can cost you even secret clearance. So for other compartments to require pay secrecy is well in line with that concern.
You can have high salary and still be indebted. Also you don't have to discuss salary to negotiate how much they would like to earn as an extra (and that may not even be money). I don't buy it, that doesn't make sense.