You're right that agency is not zero sum, but in this case we can see privilege as having enhanced agency in some areas (ability to call women names on social media with impunity, ability to command higher salaries, ability to get into better schools, etc.) and oppression as not having agency in those areas.
Resisting oppressive behavior is limiting the agency of privileged people to oppress others.
You can construct some notion of agency that doesn't say these words, but at the end of the day you're reducing the amount of available actions and that is reducing agency. Sometimes it is good to reduce agency.
This is a weak sort of agency. Indeed I'm able to resist capitalist oppression by not working, but if the consequences are that I starve to death than I hardly see that as a reasonable outcome.
I think this is an uninteresting definitional question at this point, though. If I'm misusing agency than so is the author of this piece; the important thing is that we be able to communicate, which I think is possible. If you want to further nitpick specific words to garner karma, do it with someone else.
There are lots of other things you can do to resist capitalist oppression - e.g. Unionize, or create other kinds of social support. A strawman isn't very useful.
I'm sorry you've chosen to be accusatory instead of reasonable.
Plese educate me on the difference, as (at least in this context) they seem to be interchangable.
[edit]
To clarify, consider a hypothetical total autocracy, where the autocrat makes a decision, and everyone's behavior is altered by various means. By my reasoning, the autocrat has collected everyone's agency to themself.
Maybe it's not a perfect zero-sum, as you have inefficiencies in the system, but it does seem that you reach a point at which increasing one person's agency happens only at the expense of reducing others'
Your example of the autocrat who has a magical means to alter everyone's behavior is totally artificial and has no corollary in the real world, and therefore isn't relevant to the real world concept of agency.
Resisting oppressive behavior is limiting the agency of privileged people to oppress others.
You can construct some notion of agency that doesn't say these words, but at the end of the day you're reducing the amount of available actions and that is reducing agency. Sometimes it is good to reduce agency.