Do we know the specific technical reason why the plane was grounded?
Obviously the APU would not have been able to start via the battery. I dont believe this alone is a reason to ground it.
The EAD states there was heat damage. What was damaged? I can see the battery experienced significant heat damage. Was this heat damage enough to affect the safety of the plane? Was there any other heat damage. Was that enough to effect the safety?
The EAD states there was smoke. I assume the smoke they refer to was coming from the battery and nothing else. Sure, smoke does not look good but it does not mean safety is compromised. Of course the smoke would have to be ducted outside the aircraft and not into the cabin during flight.
The EAD states there was release of flammable electrolytes. Is this the issue that grounded the plane? Was there a risk of the electrolytes bursting into flames and damaging the structure or critical systems? It is not clear.
There is no indication that I have heard that there were flames in the electrical equipment bay.
Perhaps the plane is safe? Perhaps the measures taken by the battery manufacturer to contain any fire worked? Sure, the picture of the battery with the cover removed looks bad but flames successfully contained within the battery casing does not neccassarily pose a safety risk