Hi Annie,
I wouldn't be too concerned at this point. After 6 mo. of age, SIDS is decreased dramatically and once they are rolling and have a mind of their own
it's hard to make a sleep a certain way. My little one is 7.5 mo. old and just recently started sleeping on his tummy. Not every night, but once in awhile. As long as your little one can push up on arms and even pull herself up like you mentioned, I wouldn't worry about her sleeping on her tummy. As for the article, I suppose it has happened with older babies, but it is rare.
Hope that helps!
Jennifer
Mom to Jake born June 16th, 2006