This is entirely true, but also periods of history/events in history tend to be named after the fact. They probably just called it “the Union” at the time and the historical account that we are canonically reading a translation of called it the “Union of Maedhros” because that’s how it became known.
Also, it very possibly got called the Union of Maedhros because it was a monumental and tragic failure. Maedhros might have done a lot of the legwork, but Fingon was his high king, so in a very real way it WAS Fingon’s Union, even if his only action in relation to it was giving Maedhros a thumbs up.
But post battle, when people are looking back and trying to figure out where it all went wrong, and who to blame, they are not going to take the horror of the fifth battle in Beleriand and lay all the blame for it at the feet of their famously beloved and also now dead King Fingon the Valient.
Especially if you, for instance, live in the city of Gondolin under the new High King Turgon, who was against following Feanor and his kin from the beginning and is mourning his brother. Which is exactly where Pengolodh, a historian from Gondolin who is one of the “original authors” whose works were “compiled and translated” into the Silmarillion.
This sounds kind of far-fetched until you consider the fact that we know people were thinking about this battle in terms of it being a consequence of following Feanorians, because they named it Nirnaeth Arnoediad, “the battle of unnumbered tears” after the first line of the Doom of Mandos. The Doom that also states “On the House of Feanor the wrath of the Valar lieth … and upon all who follow them it shall be laid also.” (Emphasis mine. Reason for putting actual quotes in my bullshit theories suddenly? Unknown)
So, how did everything fail? It wasn’t Fingon, it was Maedhros’s fault! It was Maedhros’s Union! That’s why it had an evil end!
I can pretty much guarantee that they would not have called it the Union of Maedhros if they’d been successful.
And I can do that by pointing at another time Maedhros brokered an alliance and then they fought a battle: The Dagor Aglareb. (which I’m super glad I double checked the name of, cause I get that and the Battle Under the Stars mixed up for some reason)
The Noldor are on the brink of civil war, Fingon rescues Maedhros, who negotiates for land in the north with Doriath, and gives up the crown to reunite the Noldor, and they fight a glorious battle and start the Seige of Angband, and they don’t name it after Maedhros, even though he did all the necessary negotiations, even though he was the main reason it happened and didn’t fail. It was just the Noldor’s victory.
