Yep, this worked in my test.
I was able to reproduce the problem without Bitvise on Windows by simply reconfiguring the sshd service on my Mint machine to have RSA + ECDSA keys. Tested before your fix and had the ‘not implemented’ exception. After the fix, no exception and Duplicati showed me the RSA key fingerprint.
So yeah I think this is a good fix when Duplicati is running on mono.