On the other hand, SSH gives you just that if you verify the fingerprint which is not mentioned in the guide. MITM-proof channel is needed if one gains any kind of shell access because then public key substitution can happen in various ways, even without the user him/herself explicitly editing the file. Furthermore, if one allows MITM when accessing shell, substituting the public key is just one of very serious security problems.
Certainly many other operations require a tamper-proof channel, and many more things can go wrong without one but I believe the author was trying to indicate the specific requirement here rather than making a specifically incorrect statement.
