They say this but it is not true
I helped someone with a locked device copy a whole bunch of stuff before she quit
Oh and also, Knox has already been hacked
In OPs case it is his own device, so it is quite simple to root the phone and do what you want to
Earlier versions of Knox weren't very secure (stored pin in plaintext) and could be hacked if you rooted the device but on the S7 it's been made much better with some elements running in hardware. Modern version use hardware to decrypt the PIN and have an "e fuse" that permanently disabled the ability to access Knox on the device if it's ever rooted or has a non Samsung boot loader installed. You won't be able to access it at all after rooting.
But you missed the point. The point isn't whether YOU could hack it with root on your phone. The point is the company monitoring software is isolated from YOUR data. And if they execute a wipe of your phone is just the Knox partition with their data that gets wiped.
I actually do the reverse and put my personal stuff in Knox. Specifically because I care more about keeping my banking secure than the company email. To get into my phone and read company email you just need a fingerprint, which can be spoofed. But to get to my banking you need to launch Knox which requires a PIN to decrypt the data. Also when I cross the border I delete Knox entirely leaving only work related material on the phone.
Nothing is perfectly secure but for me that hits the right balance between convenience (fingerprint access to work email and calendar, music, and map) and security (additional PIN to get to browser, banking, personal email inside Knox).
The way I did it it does mean the company could wipe my whole phone but I have nothing I can't rebuild there so I'm ok with that and if I lose my phone likely glad of it.