The difference between manners and morals is that the former are necessarily social in nature whereas the latter are not necessarily social in nature.
Statements
Manners/etiquette are always social.
Morals are not necessarily social (i.e. at least sometimes morals are not social)
The rules of manners/etiquette do not apply when one is alone.
Evaluate
The author has made a distinction and said a couple strong ideas.
Manners/etiquette (same meaning) are always social, so the last sentence is taking that contrapositive and thinking,
Meanwhile, since morals are not necessarily social in nature, there is at least one moral rule that isn't dependent on other people, and thus at least one moral rule probably still applies even when we're alone.
Goal
It's hard to know what to predict here, but my guess would be that it will relate to the idea that morality might still apply when we're alone.