Because it's not the text that causes it to crash. Well in this case, he attached almost 95,000 text blocks (the standard allowed number is like 3000-5000) so right htere he broke the standard settings.
Then, he tagged 1675 DIFFERENT people.
You have to understand how federation works. Like when you and I talk my instance "gets" the info from you. Your instance "gets" the info from me.
When there are three people. Person 1: get person 2, get person 3. Person 2: get person 1, get person 3. Person 3: get person 1, get person 2.
Then when they post, that repeats. Now imagine a thread with 1675 people and all the instances/users are sending requests and posts at once. It's like millions of federation touches.
It's not a software problem, its a user breaking the software problem