So your argument is that tanks and healers are the same thing. That's a fair argument I suppose.
But the definition of a tank is to "protect the party". The top way they do this is to hold the attention of enemies so that they take all of the damage, but alternative ways they can achieve it is by applying mitigations to the party.
Whereas healers are supposed to heal them when they actually do take some damage.
The lines are blurred here because of the fact healers are able to "protect the party" and tanks are able to "heal the party".