It does say "up to 30 minutes" - which would be the condition when 407/400 are gridlocked, and 413 is flowing freely.
The question should be what is the average time saving. Given that Highway 413 is 52 km, and the distance on the 407 to 400 near the King City ONRoute (where Highway 413 will meet Highway 400) is only 45 km, there are going to be times of day, where the 400/407 is faster.
(for comparison, 401/400 is about 48.5 km and 401/410/400 is about 50 km. Even 401/427/400 is only 51 km!).
That the 413 is the longest route, also means that the province could achieve all the time savings goals by just adding tolls to the 400 at the same levels they use on the 407!
The travel time projections that result in the "30 mins faster" are indeed for rush hour, but they do indicate some congestion of the 413 as well, just not to the extent that the 401 is expected to have during peak hour in 2041. The 413 is about 2km longer of a route vs. taking the 401/400 if going from Milton to, say, Newmarket, but even in uncongested periods that would likely be mitigated by higher average travel speeds on the 413 as people drive slower through the central 401 at all times of day than they would on the 413 with it's modern design standards, lower traffic volumes, and full-speed interchanges.
Even mid-day and on weekends during off peak periods, the 401 is often congested between the 427 and 400 anyway, mitigating any time savings from the shorter trip distance.
The 407 will be faster much of the time, yes, but it is tolled and a decent percentage of drivers avoid the 407 entirely, and a vast majority avoid it for daily commuting. It costs $19.48 for a passenger vehicle to make that trip during morning rush hour, for example. Trucks also do not use the highway at all, more or less.
At the end of the day, if you are going from the western GTA to north of the GTA, the 413
will result in generally positive time savings. The amount obviously varies significantly based on origin and destination, travel time, willingness to pay for the 407, etc.
The 413 has also been indicated to reduce overall vehicle-hour delays from congestion in the western GTA (including arterial road congestion) by approximately 20%, not even including the trips which are made faster by the new infrastructure even in uncongested times. The majority of congestion relief is actually expected to come from arterial roads in Peel, York, and Halton Regions, which will require far larger widening programs if the highway is not constructed. You will notice that the Town of Halton recently appealed Brampton's "urban boulevard' plan as it forces Halton Region to widen a ton more of it's arterials without the 413 being built.