It's interesting that they chose performances with very normal tempos and durations, and then this Haitink M9 is one of the longest in the entire catalogue. A bit extreme if you ask me, so I don't think it's the best choice for this being a sort of 'introductory set' to Mahler newbies. It's a very moving Ninth, but certainly an outlier in this set with almost Maazel-ian tempos (Maazel's Philharmonia recording still holds the record for longest M9, I believe, clocking in at over 95 minutes). It's funny, Maazel's tempos are so extreme that you can compile a cycle where Maazel's record breaking M8 and M9 (97' and 95', respectively) are longer than an average recording of M3 by someone else.
Rattle did a fantastic Ninth with the BPO in Taiwan in 2011 (it's on the BPO's digital archive). Maybe they were actively avoiding too many conductor repeats for this BPO cycle, and Rattle's also the only person who's conducted 7 and 8 in the last 10 years. Options were probably limited.