I have seen this in the past as well on some brazilian accounts. Those guys were and probably still are able to tag about 300 people per post.
I have spent quite a significant amount of time trying to figure this out & actually found a way to do this.
1. Crate a life event like new relationship, doesn't really matter, just make sure that it's an event that allows you to tag more than 1 person, so not something like marriage as this obviously doesn't work.
2. Only tag a maximum of 1 person and post the life event.
3. Now go to your timeline and right click on the time of this post & click on edit. Open this edit thing in multiple windows, aprox. 20...
4. Go to edit post in each of these tabs & add about 10 different friends in each of these. Do not save it.
5. Once you have added all the new different friends in all of your tabs, quickly go to each tab & just press enter. This will start the saving process.
6. The saving process takes a lot of time & during that time facebook still think that your life event just has 1 tagged person. As a results facebook will save all the different people you tagged in all the different tabs & add them together as its the same post.
7. Finished. Still works perfectly fine...