SSH never trusts the network. Even if hackers took over the entire network, all that can happen is that SSH is forced to disconnect. Hackers cannot decrypt, play back, or compromise data on the connection.
This is not to say that the SSH is perfect. Like any other tool, SSH is only as good as those setting it up and using it. For example, SSH does have an option for encryption type (none), but this is only to be used for testing purposes. (There is no real enforcement of this, of course.) And SSH does nothing to prevent someone who had gained access to the host another way (perhaps by sitting down in front of the unprotected host itself) from doing a lot of damage with root access. In that case, SSH is often the first target of a local hacker.
Why is it that in the first instance there is a definite article in front of SSH, but in the second instance as well as in other places above and below there isn't? What exactly is the reason behind placing it there in your opinion?
SSH is a network protocol (it's also the name of the popular Unix client application that implements that protocol) used for remote computer access that has replaced Telnet. The main reason SSH has replaced Telnet is that SSH makes communication over a public channel such as the Internet completely secure.