Individual politicians and political parties routinely use count a vote as approval. In that way, if no other, voting does serve to support the existing system.
But, even if you believe there must be revolution and the current system CANNOT be reformed, voting is still harm reduction, unless revolution will happen before the results of the election can influence the system.
I never claimed they would use non-voting as a signal for anything, only that they count votes as agreement, not mere tolerance.