Their Exempted Activities list does exclude Mountain Biking so that's entirely possible.
Plus, they insured a bunch of people for TCR last year. I'm just confuzzled now. I can't remember how I sorted insurance. I think I might just get a multi-trip with them to cover TABR / TCR.
ETA Permitted Activities says:
"Cycling - Standard (excluding BMX, mountain biking and racing)"
Erm..