My reading of the article is that they didn’t try to sue the guy that sold him the guns (because that fellow apparently complied with the background checks, etc.), but rather the person that sold him the ammunition.
I think the idea they put forward is that the seller should have known that selling one person 4k rounds at one time was weird and stopped the sale.
Is buying 4,000 rounds all that unusual? Seems to me that a couple friends could burn through that in a day or two at the range.