I don’t think two developers for a few years could actually have built that full game, even with procedural generation.
When they say 15-20 people and 5 years, I believe them.
However, let’s say they staffed up linearly – 3 people on day 1, up to 20 people after 5 years.
Let’s say the cost of an employee is $7k per month – this is European or Alabama salaries, not California or New York City salaries, plus overhead/taxes/equipment/whatnot.
That gives a cost of (3+20)/2712*5 == about $5M to develop.