The article states they raised billions to allow employees to sell their (otherwise soon to expire) RSUs to investors to cover their tax bill. Stripe itself isn’t using the money.
Often plants will take up the toxin through their roots and sequester it in their aerial tissues. In this case the tissue could be harvested (and then perhaps incinerated) and the pollution will be removed from the canal. If aquatic plants were used and they didn't fully degrade the pollutants, then the waterway would be still contaminated.