You cannot and must not evaluate it quite that simply. Even in property purchases "as is," there is the obligation that the seller must voluntarily point out defects or value-affecting factors known to them. In this case, the circumstance to be assessed is whether the seller knew about this soil contamination and possibly deliberately concealed it to achieve the expected purchase contract at the hoped-for price. If this is the case, the contract could possibly be contested due to fraudulent misrepresentation.
As always, the "double intent" must be proven here. This means the seller’s knowledge of the special condition and their concealment of it with the aim of concluding the purchase contract, which would not have been concluded or not at the hoped-for purchase price if mentioned.
Referencing a soil report is always helpful (without one, I would never build), but it is not the only way to handle such matters.