With basic means (i.e. software to write to the memory card) I am rather doubtful that it is possible to trustedly delete all sensitive data.
Inside of such a SDXC there is a complex microcontroller. Those microcontroller is not known to you and its software/hardware is proprietary. The memory size shown by the SDXC must not reflect the complete data the microcontroller can write to. Even likely there is flash memory for 70, 80, maybe 128GB of flash memory in your device.
At first this would look like an extremely wasteful way of memory usage (having 128GB worth of flash memory and only offering 64GB). However it is less wasteful considering that a fair fraction of the flash memory is defective/broken already as the outcome of an ordinary production process.
The real deal is that under the hood such a memory is pretty f***ked up. The microcontroller is to counter the messy "under the hood" by using techniques of error checking to give the appearance that the data on the SDXC card appear to be safe and guarded, while internally memory cells are dying.