How can I be sure my information on that phone truly is erased

My phone received water damage. I took it to the nearby Apple Store and was going to replace the phone under AppleCare+. I’ve done this before. Standard protocol is to to erase everything on your iPhone before turning it in. Well that’s where my problem begins.

I was asked to turn off find my iPhone and to also go to settings and choose erase iPhone. I immediately told him I can’t. My phone is fried. My phone won’t turn on nor will it restore in iTunes. iTunes gives me an error message saying it can’t restore it.

He then proceeds to give me the new phone and assured me all the information that is on my broken phone can never be recovered. I then asked for a guarantee that even though we couldn’t format my phone then and there, all my information will not be hacked. He said since your hardware is damaged not allowing a system restore there’s no way anyone can recover my lost data from this phone.

i turned in my phone, left with the new one, and went on my way. Here’s my problem, how can I be sure my information on that phone truly is erased (damaged, corrupted, etc) so that the next person/company who gets it can’t retrieve any info. Remember, we never did a format on my phone, we couldn’t, therefore I never got the satisfaction of fully knowing that my phone was in fact erased. The only thing I left the Apple Store with was the assurance of an employee.

Please help.

-your loyal Apple family member from Texas

[Re-Titled by Moderator]