If the tech opened it up and noticed a third party battery, they wouldn't have proceeded to remove it. It almost certainly was a genuine battery in that case.
I have been working in phone repairs for the last 15 years. The only times I have seen a battery overheat/explode/smoke, whatever you want to call it, is upon removing an iPhone battery. iFixit make it look easy with the pull tabs, but let me tell you, once that battery has been in for 1+ years, those pull tabs aint working! You usually need to wedge things under and round each side to break the adhesive (Use a flexible opening tool etc). That's when you puncture the battery, or you have to deform it pretty extreme to get it off the old adhesive. I personally experienced it twice in about 500+ batteries replacements, but there are some that puncture and nothing happens as well. I imagine this being an iP6, it was a PITA to remove and they used a tool to try help remove it and it punctured it.