It's not. Not even when used as a suffix in 'real' words.

https://www.merriam-webster.com/dictionary/quintuple

In thinking about this, I realized I say short quintuple but long standalone tuple. I can probably retcon myself some reason but I don't think there is any.

You should use 'tupple' when referring to odd-length tuples, and 'toople' when referring to even-length tuples. That is obviously the only sane compromise.