tiawd –
I think the Tanya was well aware of ideas we are not allowed to believe and nevertheless wrote what he wrote including the word “Mamash”.
As explained here above, once the principles of אין עוד מלבדו and the true meaning of Hashems unity are clear the Tanya’s idea is very understandable.
If you feel you have more to add than what was described here to explain what חלק אלוק ממעל or the balloon mashal brought in the name of the Zohar means, you are more than welcome to do so.
Most important is to not even come across as undermining this powerful idea or it’s implications that inspire thousands of Jews, even during an attempt to further refine and clarify a more exact definition of the idea.