However, the definition of a mathematics major is, I think we'd agree, "someone who has majored in mathematics". So, regardless of how much of the corpus we manage to cover, having a degree in mathematics means that you majored in mathematics (if I am understanding US terms correctly).
The original post was, I think, equivocating a degree in computer science with being a mathematics major. Which could be regarded as being incorrect, perhaps pedantically, merely by definition alone.
However, also, I am making a distinction (although I wasn't very clear, I admit) between theoretical computer science (the topic) and a computer science degree. It is generally accepted that theoretical computer science is a topic in applied mathematics, but it doesn't then follow that having a computer science degree means that you are a mathematics major.
Computer science degrees have theoretical computer science (the topic) as a component; the extent to which that component makes up the degree syllabus varies greatly from university to university and, perhaps, has also changed over time.