He means that it will be hard for the water to get out of the tube when you try to water it. Leave some space between the tube and the bottom so the water can get out when you pour it in.
Why use peat and perlite in an even mixture? That seems okay for neps, but sars like to be wet. I use straight peat for mine, always have. That way I don't have to water so much. And I'm in an area where we get plenty of rain most of the time. They stay pretty waterlogged and seem to enjoy it.