In general you get 1 point per £1 spent so if you spend £100k you get (at least) 100,000 points.
But some transactions reward you with 2 or 3 points per £1 spent (foreign currency transactions, airline ticket purchases, and Amex travel site purchases) and and you get bonuses for meeting certain spending thresholds as well as referring others. So I imagine people who accumulate 100,000 points will generally do it with a far lower spend than £100k
Sign up via a referral bonus and spend at least £3,000 in first 3 months: 12,000 point bonus
Spend at least £15,000 in 12 months: 10,000 point bonus
Refer 8 friends: 8 x 6,000 = 48,000 bonus points (can earn up to 90,000 per year this way).
£5k in flights purchased direct from airlines = 10,000 points
£20k in general spending = 20,000 points
Total = 100,000 points.
Amex have reduced their bonus rates in the last few months so someone using the card 12 months ago would have benefited from more generous bonuses than used in example above.
The £500 per 100,000 points comes from the value of each point if converting to gift cards or some other forms of pseudo-cash.
But points can be worth more if converting to airline miles or hotel points so can get £500 worth of these types of rewards with less points.