tiawd –
Correct, in לשון בני אדם or what you call day-to-day level the Neshama is a creation not a creator as it says in אלוקי נשמה. However, the point of the Tanya is that the Neshama is a different kind of creation than all other creations due to the fact that as opposed to all other creations, this creation has a part of the creator in it which means Hashem restricted / covered himself in this creation less than in others.
The explanation / mashal given is מאן דנפח מן דיליה נפח a balloon full of air and an empty bag are both creations of their creator but the balloon has a part of it that is part of the essence of it’s creator / one who blew the air into it as opposed to the empty bag which is totally separate from the one who created it.
Once we all agree that אין עוד מלבדו, “kefira” is hopefully now out of the question, the issue now is how to best describe / understand in לשון בני אדם the uniqueness and specialness of the Neshama as opposed to all other creations that the Tanya is teaching us.
I see no problem with the word “Mamash” being literal in the context of the fundamentals explained above.
I would say a lot of the discussion here on how exactly to describe the idea of the Tanya is what Chazal call משמעות דורשין איכא בינייהו.
More important than how to describe the exact “anatomy” of the idea of the Tanya are the repercussions of the idea, such as proper self esteem, proper view of other people, hopefulness, optimism, belief in Teshuva and more that result from this idea as explained by the Tanya throughout his Sefer.
It would be a big pity to miss out on the main point/s the Tanya is trying to help the reader achieve by getting stuck on the exact “anatomy” of the idea which either way you go as we explained, is not even remotely close to kefira.