Yes, it seems to work as expected (i.e. identically with both OG and sudo-rs) when the are lines split, in either order, as long as timestamp_timeout value is unquoted. Double-quoting it again causes the ”expected nonnegative number” error (regardless of whether that line is first or second).

$ sudo-rs chown jani:jani test
/etc/sudoers.d/90-ssh-auth-sock:2:39: expected nonnegative number
Defaults!/bin/chown timestamp_timeout="0"
                                      ^
$ sudo cat /etc/sudoers.d/90-ssh-auth-sock 
[sudo] password for jani:
Defaults!/bin/chown env_keep+="SSH_AUTH_SOCK"
Defaults!/bin/chown timestamp_timeout="0"