why is it still called secret legend on the "window" title

[LISTERINE® ANTISEPTIC Mouthwash]

wondering if its on purpose or if its an oversight (doubt it though)

[Om nom nom]

It is on purpose. It was the original name for the game and it still exists in many of the actual game files if you look in the game folder. The game that the Heir played, other players, and the fox that we play as, are playing the game "Secret Legend". Hopefully that makes sense. "Tunic" is what we think the game is called at first, and of course it is. It's also "Secret Legend".
If you did the 2nd ending and pay attention to the end credits, this might make more sense. Plus the hand written notes in the manual is a hint that we were not the first to play the game. The Heir played before us and was the previous player and all of the dead fox bodies were previous players. It's meta.

So yes, it is definitely very much intentional that it is still referenced as "Secret Legend".
The reason it is called Tunic is the tunic that the fox wears is a clear nod to The Legend of Zelda. So is Secret LEGEND. Both are nods to Zelda. This game is a love letter to the The Legend of Zelda and heavily influenced by the older Zelda games.

The whole in-game manual is a love letter to old school Nintendo Power issues well before the internet was even invented. Tunic is nostalgia done right. Tunic is the only game that has an in-game manual be such a huge part of the actual game. It's truly brilliant.

[Om nom nom]

That is a LOT OF SPOILERS you've got there, mate. >.>;

The OP question itself is a spoiler. in the headline post itself.

"why is it still called secret legend on the "window" title
wondering if its on purpose or if its an oversight (doubt it though)"

Maybe the OP should edit their original post to mention that this entire thread will be SPOILER talk considering the question itself is an actual spoiler. It's only possible to understand what the original question in this thread post is talking about after you figured out how the language works and translated the manual.

I was mostly answering the question by referring to the game's influence rather than actual spoilers of the game itself.

If the OP reads this maybe you should edit to mention that this entire topic is spoiler territory.
I will edit most of my post to just black most of it out then.

[Om nom nom]

That is a LOT OF SPOILERS you've got there, mate. >.>;

There. I edited my post to cover most of it but your quote still shows it all uncovered. You will have to even edit or delete your post to remove it.

[Searanox]

Originally posted by Om nom nom:

That is a LOT OF SPOILERS you've got there, mate. >.>;

There. I edited my post to cover most of it but your quote still shows it all uncovered. You will have to even edit or delete your post to remove it.

Done. Good work.

[Xenustik]

It's funny, but the name "Secret Legend" was what first allowed me to start deciphering the language by looking at the title screen of TUNIC.