It is, though I'm not certain it means much. In fact it's misstated since it's the probability that this exact sequences appears in 30,000 uniformly random base

It is, though I'm not certain it means much. In fact it's misstated since it's the probability that this exact sequences appears in 30,000 uniformly random base pairs and it appears in a library of 24712 sequences of sequences of 3300 uniformly random base pairs (see Figure 2). Which to me doesn't say much. It's like pointing out it's very unlikely it would have been typed by a monkey mashing on a typewriter. A somewhat more honest question would be to look at what they did, which is look at a length 12 sequence (and it's complement) that they apparently found interesting and see if it showed up in a library of 24712 sequences with an average length of 3300. You can estimate this probability by calculating the average number of matches you'd expect to find. This can be done by taking the number of positions a match could be in, approximately 24712*3300 - 24712*11, times 2 to account for its complement showing up, and multiply this by the chance two length 12 sequences will match, estimated by them at (1/4)^12 (in reality this probability should probably be higher since base pairs aren't uniformly random). Since this is an expected value we can ignore that matches at adjacent positions aren't independent. This suggests you'd expect to find about 10 matches. So I'm not sure if they simply picked the longest one or if there was just the 1 match. You do get a lowish number of 0.000059 of matching sequences of length 19, that said this chance would be significantly higher when you know there exists a match of length 12, especially if the sequences aren't simply uniformly random.