This is not the case, from the rulings on Contamination (emphasis mine):
External effects that generate more mana still generate their additional mana as directed without being affected by this card. Effects which generate more mana based on what type of mana the land produces will be affected in color but not in amount.
You missed the first two words of your quote: External effects
This specifically refers to effects such as Fertile Ground and Vernal Bloom, which can cause more mana to be produced beyond Contamination's limiting effect.
Cabal Coffers can produce a maximum of B with Contamination, but Contamination won't stop Fertile Ground from then producing more mana.
You missed the first two words of your quote: External effects
This specifically refers to effects such as Fertile Ground and Vernal Bloom, which can cause more mana to be produced beyond Contamination's limiting effect.
Cabal Coffers can produce a maximum of B with Contamination, but Contamination won't stop Fertile Ground from then producing more mana.
Also, this is from 2012, so
No longer staff here.