I'm really surprised that so many people aren't getting this. People are frequently written out of history. My generation learned about Florence Nightingale at school, but only in relation to her legendary status on the battlefields of Crimea. It took me decades to catch up with her seminal work in epidemiology and data representation, and her reforms of nursing, which changed the practice of public hygiene forever. She was far more than just "the lady with the lamp."
Meera being written out of the history of the war of the five kings makes sense. She is an unsung hero that no one will remember in history:
Same with people like Gilly, Podrick, and Tormund.
It would be egregious, but you could even see someone like Seleyse not being mentioned in a history of the war, if the historian focused on Stannis and his red priestess and didn't feel the need to name who his queen was.
But Tyrion is a CENTRAL figure to the entire war. You cannot tell the story - you literally can't describe the sequence of events - without once mentioning his name. It's impossible.
How do you even describe the start of the war without mentioning that Cat abducted the queens brother to charge him with murder which in turn caused the kingslayer to attack the hand of the king, murder his men, and cripple him for life?
How do they mention the death of Tywin Lannister without mentioning that he was killed by his son, the imp?
How do they talk about princess Myrcella being poisoned in Dorne without noting the hand of the king that sent her there.
How do they mention the death of the king at his own wedding without noting who was charged with the murder? How do they mention the Dornish prince being killed during a trial by combat without mentioning what the trial was even about?
How do you talk about Sansa Stark being married off without mentioning that she was first married to the Lannisters?
How do they mention the invading Danerys Targaryen without once noting who her hand was?
Tyrion is one of the most popular people in all of Westeros, even before he did anything notable. Arya was dying to see "the imp" in the first episode. Cat knew of his reputation before she had even met him. Oberyn described in season 4 that everyone was interested to see him when he was just a baby because he is perceived as this monstrosity. He is basically a Westerosi celebrity, infamous and hated.
But most important, he is directly tied to the story of the war.
Nothing about this "you aren't mentioned in a 5000 page book about this ten year period of war" makes a lick of sense. It was only put in there for a joke. There is nothing about it that rings true or acts as a parallel to history.