Snowden lying about something as easily verifiable as his salary indicates that he might not exactly be the most trustworthy source.

If he was willing to lie about the salary, what else was he willing to lie about - up to and including the kind and type of information he claims to have had access to, the problems with the information he saw, etc.

It goes to the character and judgment of the suspect/witness.

It’s not like he said that he was getting paid $200k and he was really getting paid $185 or $190k.

He was off by a whole lot more than that. Entire pay grades different.

Why would he lie about something like that? To brag? To claim importance that was neither deserved or warranted?

These are all important questions about the provenance of the source of the story.