Admin backup/restore does not support SFTP, it supports a secure FTP, i.e. FTPs.
SCP is more alike SFTP. Unlike SFTP, SCP has fewer commands and is used primarily to copy files between a local host and a remote host or between two remote hosts. SCP is unable to create or list directories or delete files like SFTP.
I don't use the System backup over FTP/SCP so can not say anything specific. You might test the system backups on your own and let us know your results.
You might check and update scripts/settings under the directory: /usr/local/sysbk