Well, the article is referring to her father’s fanbase, not her own. I don’t know if John McCain’s had any approval ratings taken recently, but their source was probably that traditionally this is the voter’s he’s drawn - liberal republicans and independents. I’m not sure how “devoted” they are now (or ever were) but that does fit the demographic that supported him in the last campaign (plus conservatives who once again felt the need to plug their noses and cast a vote against Obama).
As for Meghan McCain, I have a hard time believing much of anyone cares what she has to say. This article is only worthwhile to the extent that it’s the sort of thing to make Team Romney squirm as they try to cast their candidate as an actual conservative.
Not true. Look at the full paragraph I excerpted--the reference is clearly to Meghan McCain:she is the subject of the previous sentence, and the one I excerpted doesn't then say "And her father..." or "And Senator McCain..." So it's to Meghan.