Boeing did not make the call, Thales did and told the airframer that the battery used on the 787 was safe with proper handling of charging/discharge currents, temps, and the so-called fail-safe electrical integration. They were wrong and the NTSB investigations clearly showed this.
The failure is a perfect example of how outsourcing can go awry. Never take the word of another manufacturer - test and re-test yourself. (Not an easy task in the complex and electronics/software-heavy world of commercial aviation.) It was also illustrative of the dysfunctional certification process that fully approved the 787 batteries in the first place.
Lithium iron phosphate batteries can overheat and outgas and fires are not impossible in the battery assembly, just unlikely. The manufacturer of these batteries includes aviation firefighting strategy guidance should their batteries catch fire. Never say never is the point.
If the Iron Phosphate cells are perfectly safe, Airbus should have no problem testing them for the overcharge condition - satisfying the now-skittish FAA's requirement.