Some stuff in the article wasn't really explained but I guess if I were a PFF fan I would know what the hell they were talking about. I have no idea what downgraded is but I'm guessing being third lowest is a good thing.
All I really got out of it is he good.
PFF grades each play from -2 to +2 as many coaching staffs have traditionally done. -2 is a very bad play, maybe blowing an easy tackle or falling down resulting in an 80 yard play. A +2 might be a pick or forced fumble. A 0 grade is where the guy does what PFF perceives to be his assignment but the impact on the play is neither good nor bad.
When they say Player X was downgraded the 3rd. fewest times at his position, that means PFF gave him the third fewest -1 or -2 rated plays. I'd guess PFF is doing that on a percentage of snaps basis, but I couldn't be sure unless they say so explicitly. What that means is he does not make many mistakes.
Amos has not, on the other hand, been racking up a lot of +2 plays compared to others, typically measured by INTs, forced fumbles or pass breakups.
He's a steady Eddie, assignment-sure kind of guy. Balls don't get over his head and few tackles are missed. He's the kind of guy who might not get Pro Bowl attention for lack of picks, which always seems to be the guage together with reputation, but he's the kind of guy who should make his DB partners better.