Low cadence, high torque training helps MTB a lot IMHO. When people mention having low back tightness and fatigue during technical climbs and fast MTB efforts, they are actually referring to hip and core instability due to lower-than-training cadence rates and higher-than-training pedal torque. The higher forces in these pedal strokes are translated into higher forces placed on your hip stabilizer and back muscles. This is also combined with having to move the bike underneath the rider while at the same time maintaining balance and power, which creates a nightmare for an unconditioned low back. Unless you are training these, I think you are asking for trouble.
For instance, look at MvdP the past 2 weeks. I don’t think anyone would argue he is putting in a lot of training hours and incorporating strength training, but he has openly admitted his back is not in shape for MTB efforts.
Just my $0.02