Because having two candidates that have so much in common they should be on the same ticket means any kind of change from the status quo. So yes, if the candidates have no discernible difference, not voting is a more effective way to voice your displeasure with the status quo and influence a change in mindset.
The political class doesn't know that you didn't vote in protest, or that you didn't vote from apathy.
Not voting sends ZERO information to the people you didn't vote for.