Not at all, because secular humanist ethics can be derived without any reference to theistic concepts. That one can not get there by pure reason alone is a non-sequitur, as that applies equally to theologically-motivated ethics.

Furthermore, let's suppose a theologian - for the sake of example, let's say a christian one - insists that I am really a christian, on account of my ethical positions. Well, OK, but if that so, then it follows that being a christian does not involve believing in the Trinity, or that Christ is the son of God, or that God created the universe... in fact, that it does not involve anything beyond holding certain ethical positions, and that christianity is identical with secular humanism!

Of course, it is rare to find an avowed christian who follows this line of thinking to its logical conclusion. Motte-and-bailey argumentation [1] is rampant in theological thinking.

[1] https://en.wikipedia.org/wiki/Motte-and-bailey_fallacy