1. New tensioners do fail as well... the garage may have done everything 100% and the new tensioner may still have failed.
(my Vx dealer had to re-do the whole cambelt kit the day after they installed it because the new tensioner was noisy out of the box!)
2. But - some crank pulleys had rubber vibration dampers, if the rubber fails then the inner part of the crank pulley will rotate freely and independently of the outer part, the result is similar to a tensioner failure only the belt does not actually snap!
3. On balance the pulley separating seems consistent with a low-mileage older car - it's the ageing and the sudden load of the engine starting that kill the rubber, not motorway mileage.
4. Whether it's the tensioner or the crank pulley, big end problems on a 89k V6 is almost unheard off (unless the engine has been seriously abused/neglected?), and the symptoms are in-line with some sort of upper valve problem. The bad news is that if the valves hit the pistons - for whatever reason - the engine is very likely to be a goner...