When presented with two valid routes for an object, buildcraft pipes will approach this problem probabilistically. If there are two routes, 50/50 chance for either route. The exception to this is when you have a diamond pipe with a specific route AND a generic route, it will prefer the specific route but take the generic route if the specific route would (immediately) fail. So a diamond pipe can be part of an insertion group.
you can actually use this approach to create a crude manual sorting system temporarily. my guess is he's doing what i did and avoiding a m.e. network because he doesnt have reliable power yet, opting instead to quarry resources to allow a more robust starting power system.
i actually used this approach, diamond pipes to send cobble/dirt/gravel/marble to dsu's, obsidian/coal/flint to specified chests/ and cobblestone pipe probability to send everything else to 10 full sized wooden chests. it was enough storage for a full size quarry from y=50, and i'd just manually compile and sort it before moving the quarry, which took about 30 minutes
sometimes, people spend 5 hours planning something intricate when you can just kludge your way to the resssources for a more efficient base setup. one day of spelunking plus charcoal = quarry rush on day 2.
side note, this site does not like my phone keyboard or andriod 4.1, dont shoot the no caps, shift doesn't work nor does autocomplete.