It's hardware decryption. The key only ever exist within the SOC. Throwing away the decryption key means overwriting it with a new one. There is no possibility of recovery.
If this hardware encryption/decryption is trustworthy, then what is the difference between it and TPM, which few data experts are willing to trust?
"Zeroing the storage space" probably does not overwrite anything on flash storage. Flash is very resistant to writing anything to a block unless it has to, as there are limited numbers of writes before the the block becomes unusable. Writing random data will, but at a cost of significant time. And it's still less secure than deleting the key of an encrypted drive.
I have recently been playing with hdparm and ATA secure erase and enhanced secure erase. As I understand it, issuing the command for enhanced secure erase returns the drive to a condition defined by the manufacturer of the device, presumably one which does not retain any data. Additionally, I found a blog post by Bruce Schneier discussing a report from a trusted security company which stated that traditional full disk wipe methods for HDDs are also effective on SSDs. The notable exception is that the security company did not find any delete-based wipe methods effective on SSDs (meaning, you have to wipe the whole disk to completely erase data). That last bit annoys me: everyone is so concerned with deleting data on far-away devices, yet we can't even delete specific data on local devices without wiping the entire device.
I have not played around with wiping data from phones, so I don't know how any that applies, but I suspect the concepts are the same. Also, as far as the time component goes, it's unattended time, so little measurable cost to the user.