I can advise someone against voting now even if I would advise them otherwise once fewer people were doing it.

Consider a travel advisor. They suggest you visit remote location X because the people there like foreigners but it's not too touristy. To one person, this is good advice. To enough people it is bad advice because once they get there they will find that actually it is quite touristy.

The reason that "sound ethical advice directed at an individual is irresponsible when directed at the aggregate" has some truth to it is that it's very hard to carefully explain the complexity of how in the current circumstance something (not voting, professional philanthropy) is the right choice for one more person to do but that if a bunch more people do it then other choices do better.