Salamun Alaikum,
There are numerous concepts of God and even today false deities are worshipped as they were at the time of the Quran’s revelation.
In the context of a statement which is intended to provide a response which categorically denies all other Gods, I feel there is nothing wrong in the human expression of ‘True God’ implying, “He is the only true God as all else that is worshipped is false”.
It is human expression to state a condition and the expression is correct given the implied context. There is no harm in using it in my personal opinion.
I hope that helps, God willing.
Regards,
Joseph.