The reason you don't see his code explicitly stating anything about MSN is because he is using a class to store the bots that he wants to rate limit. The line that says:
if { [matchclass $user_agent contains $::Crawlers] } {
is doing a lookup in a data group (class) named Crawlers and is limiting any request with a User-Agent matching those strings. That being said, this is a very old approach to this solution. If you are in need of a bot rate-limiting rule and are on a version newer than 10.0, there are more efficient ways to go about this.
Colin