My brother said it was hail the size of golf balls falling in his neighbourhood, all the car windows are smashed. They should have made it more clear in the warning but I think it was a valid reason

When you get such an alert, there is always more information on environment canada's website. As OP said, it's mostly due to the hail. Full text of the alert on environment canada's website:

At 3:39 p.m. EDT, Environment Canada meteorologists are tracking a very dangerous thunderstorm capable of producing very strong wind gusts, up to baseball size hail and heavy rain.

This severe thunderstorm is located near Mud Lake, moving east at 60 km/h.

Hazards: Baseball size hail 90 km/h wind gusts Tornado possible

Locations impacted include: Ottawa, Gatineau, Gloucester, Orléans, Shirleys Bay, Lac Deschênes, Aylmer (Gatineau), Kilroy Crescent, Mud Lake, Nepean Bay, Hull, Dows Lake, Pointe-Gatineau, McKay Lake, Vanier, Vars and Sarsfield.

This is a dangerous and potentially life-threatening situation. Extremely large hail can smash windows, destroy property and vehicles and cause life-threatening injury. Strong wind gusts can toss loose objects, damage weak buildings, break branches off trees and overturn large vehicles. Remember, severe thunderstorms can produce tornadoes.

Severe thunderstorm warnings are issued when imminent or occurring thunderstorms are likely to produce or are producing one or more of the following: large hail, damaging winds, torrential rainfall.

I've seen them for tornado warnings, but not thunderstorms. Although the Environment Canada alert does warn that severe thunderstorms can spawn tornadoes.

If you abuse the alert system, people will just ignore it or turn it off.