I think you're misunderstanding the previous comment. It says the objective of the monogamous strategy is to increase the chances of survival for a particular set of offspring, not a particular type. That is, if a man partners with a woman and provides for the children they have together, he increases that set of offspring's chances of survival.
The point about marriage and/or monogamy benefitting lower status males is that, in a polygynous system, all or most of the females will be having sex with the few high-status males who can provide for them and their offspring (barring rape). High-status males would have lots of sex partners and opportunities to produce offspring, low-status males would have few or none.
The point about marriage and/or monogamy benefitting lower status males is that, in a polygynous system, all or most of the females will be having sex with the few high-status males who can provide for them and their offspring (barring rape). High-status males would have lots of sex partners and opportunities to produce offspring, low-status males would have few or none.