So I looked into this a while ago because I thought it was nonsensical... and it is nonsensical (as you've pointed out there is not knee pressure here).
However! If you look on page 26 of the rules:
http://ibjjf.org/wp-content/uploads/2014/05/20140507_RuleBookIBJJF_v3.pdf
and look at the picture in the second row, on the left, you will see a picture that similarly shouldn't be a reap, but is considered reaping (and is ibjjf illegal).
It looks like (according to the rules), you don't even need to triangle your legs. If you backstep, and then stick your thigh under the hamstring then you could be called for reaping. There is a bit of wiggle room in the rules (they first say you need the foot trapped by your arm, but then in the next paragraph they say immobilized doesn't necessarily just mean trapped by your arm), but I could easily seem them calling that... and as soon as you attack any feet, they definitely will!