Yes, tanks tend to be hard to kill but being hard to kill is not what makes you a tank.
Tanks are better defined by their ability to easily CC, protect, or control space (especially with their basic abilities). There are plenty of bruisers who are harder to kill then tanks and some tanks who are squishier then bruisers.
For example neither ETC and Anub aren’t all that tanky but they are unquestionably still tanks because of the above mentioned characteristics.
While heroes like Artanis, Sonya, and Leoric are very poor tanks as while they are nigh unkillable, they have very limited tools for doing the real duties of a tank.
This is also why deathwing, imperius, maiev and uther are occasionally run as tanks or have been ran as tanks in the past.
I also want to point out that having a good engage does not make you a good tank. There’s someone in every role whos got a good engage.
Bruiser is more defined by their ability to dive/pressure the enemy backline in a fight and usually also their ability to take the solo lane. They also have generally good sustain, often better then the tanks and they also generally have good damage output. In a way, you can think of the bruiser role as a bunch of big stat stick heroes with good numbers all around and tend to be very self sufficient.
There is some amount of overlap between the 2 roles and even a couple tanks can build to fill this role so it can be easy to confuse the two. Funny enough, blaze in particular is considered a better busier then he is a tank because of how good he is in lane and due to his E not being a reliable enough CC.
I say Chogall leans significantly more in the bruiser direction. His biggest weakness as a tank is very meh CC. Q sux and while his upheval is like top 10 engagement tools in the game, you don’t get it till half way in to a game and thats not what defines you as a tank as every role gets good engage tools in various heroes.
Cho’s hammer and Gall’s stunny balls are the closest he gets to being a good tank. However while hammer can be great for protecting people and keeping enemies at bay with, its pretty much all Cho’s got and you don’t get it till 10. Stunny balls are something every tank in the game would love to have but you don’t get them until 16 and that’s way to long to wait until you have a tank when waiting until level 10 is already pushing it.
As a bruiser, while Chogall isn’t quite fit for the solo lane (mostly because hes 2 players and its not a solo lane anymore. otherwise hes got a great kit for being in the solo lane) hes got every other bruiser thing covered. Almost as mobile as genji, lots of damage and hard to kill all make him idea of pressuring a backline if not outright one shotting them. Yrel is another good example sense when she was released, they wanted her to be a tank but she ended up being a better bruiser for similar reasons.
All that being said tho, I still think chogall should be treated as a tank in qm match making. Not because he is a tank but because dealing with chogall if you dont have a tank would be a nightmare.