The book isn't taking -V/rThe rate of filtration is being taken out of the volume of the pool. Why would the book choose to use the ratio -V/r?

The problem here is you are assuming the pool is being drained and no water is being added.

The book is assuming that pure water is being pumped in at the same rate the pool is being drained.

Therefore the volume is a constant 60,000gallons.

dq/dt = -q (r/V) = -q (200/60,000) = -q/300