Nah, I think Oda's intention in this chapter was to emphasize that even though Garling doesn't have red hair like Shanks and Shamrock, they could easily still be his children, because they inherited their red hair from their mother. The same way Ace's freckles came from his mother, not Roger's, and Usopp's nose also came from his mother.
And there's no denying the resemblance between young Garling and Shanks.
Now, if the mother had been revealed not to have red hair, we'd have a serious argument.