I don't think an app has the kind of low level access required to actually wipe something from memory. Unless android has an API with a function to do so. I assume you are just telling the OS to delete the file and the OS does that however it normally does that. Truly wiping a file from storage requires overwriting it with random data.
That doesn't seem to be the case here, because the files are actually deleted once the user logs out of Snapchat. So it's obviously the Snapchat app itself that is triggering the file deletion, not the OS.
With wear leveling and modern filesystems, it's harder than that actually overwrite the file on disk (for modern definitions of the word disk). The operating system will ask the storage device to delete the data, but there aren't actually guarantees that the storage device actually listened to the operating system's request to delete the data.
Which is why if you care about sensitive data not being recoverable from failed media (eq. banking database, PII, mediacal records, etc.) you should always use full disk encryption (LUKS, bitkeeper, veracrypt, etc.).
That way anything one could recover from that device afterwards would be effectively random noise to them without the decryption key, regardless how the firmware of the device actually wrote the individual data blocks.
