Just because it's zoned doesn't mean it gets built. For example, some years ago the city increased the zoned capacity of all the single family zones by permitting laneway houses, affecting some 60,000 lots. At present rates of construction, only 1/8 of zoned capacity will actually be built upon over the next 20 years. This makes sense, since not everyone wants a laneway house, just as not every apartment owner is going to want to tear down and rebuild to the maximum permitted. All of this adds up to mean that the actual capacity that people will want to develop is less than the legally permitted amount.
Further, is the zoning in question the zoning we want to see? To retain exclusionary single family zones un-touched, maxing-out zoned capacity would involve the teardown of many of the city's most affordable rental buildings in order to build to the zoning maximum. Some would find it rather perverse to direct growth towards tearing down the homes of lower income renters when homeowners could be striking mutually advantageous deals with builders instead.
Lastly, the number of people who will move here is not some fixed constant. It's affected by the availability of housing, and (all else being equal) if housing is more expensive fewer people will move here.
The claim that there is enough zoned capacity usually refers back to a report by Coriolus Consulting produced for the City in 2014. To start, the report itself acknowledges that roughly half of the 'zoned capacity' is projected to come from rezonings, which, ironically, means that there is not enough zoned capacity to meet twenty year projections at the present time.
A detailed examination of the topic can be found here