It should be a comment but It seems I have enough reputation yet.
I have no knowledge how the space is taken, but echo -en > .thumbnailsX-xxx (*Anyone trying this should do it at your own risk. *) DOES set the size to zero and total free partition space LOOKS increased, while simply deleting files can't.
An educated guess would be these files are located ...
Unless you have a means to power the device on and wipe the data properly, you need to physically destroy it. The data lives inside an eMMC chip which is conceptually just a SD card soldered to the motherboard. It will not be damaged by salt water (suggested in another answer) or other means that do not physically break the chip into tiny bits. Unless you ...
In case the phone still boots in recovery mode (usually hold Vol Up and Power-on) and you have root access you could run the command:
dd if=/dev/null of=/dev/block/by-name/userdata
This will put 0x00 to all your userdata.
As you said, the risk of drilling the battery with a drill is too high.
If there is no power at all, you have to open ...