The strategy section for Slaanesh suggests playing Perverse Infiltration cards for easy points just as regions are ruined.

The Corruption phase (pg. 19) specifically says 'each player who placed at least one corruption token in that region during this Corruption step immediately scores the number of victory points indicated for ruiners'.

Perverse Infiltration will place the corruption token during the Summoning step, not during the Corruption step, therefore this token would not count for ruiner points.

Which is wrong? Or is the strategy simply suggesting trying to sneak into first or second place for the ruination points? (Seems unlikely, since the extra one token is hardly going to lead to a surprise first or second place, chances are you were there already.)

While the rules indicate that it should only be for Corruption placed this "step" I think they mean this "round." The point is to be a part of the crew that tipped the scales when this region gets ruined (most likely to encourage players to act more quickly and jump on the band wagon). I think they overlooked that this card, and Nurgle's upgrade, allowed placement of corruption in the summoning phase. But they should be considering for contribution to ruination in that round.

Definitely needs clarification though.