Here is a link to an article in which William Lane Craig explains and defends his proposal. https://www.reasonablefaith.org/writings/question-answer/could-adam-have-been-heidelberg-man/ I think it is a good idea for me to let Dr. Craig speak for himself. You will see that in this article Craig admits that his idea is quite speculative, based on assumptions about artistic and language skills (which we do not know) as well as brain size (which we do know). He makes it clear that he is not trying to find “the answer,” but instead to start some helpful discussion. You should bear in mind that Craig is not a scientist. He is a philosopher. My response to this is that this is an intriguing possibility and I would not outright dismiss it, but it is merely a hypothesis and a speculative one at that. But, yes, I would say that it is a reasonable hypothesis (Note: Craig calls it a hypothesis, not a theory because of its tentative nature) I think it is good that Craig is engaging in this discussion. To me it is more important to have an intelligent discussion about what the first persons created in the image of God would be like than to try to find “the answer” to this question, given that we simply cannot know. Here is my suggestion. If you are talking to someone who wants to know “the answer” to the question of human origins (rather than engage in a discussion and leave room for doubt), I would say to this person that we simply do not know for sure, and perhaps not present this fairly speculative theory. But, if you are talking with a person willing to engage in an intelligent discussion, but not looking for “the answer,” then it would be a good idea to introduce them to this discussion and hypothesis from Dr. Craig.