Context is important.
A/T == 1 and G/C == 1 (approximately, to within a measurement error) are Chargaff's rules for double-stranded DNA, occurring because A pairs with T and G pairs with C.
(A+T)/(G+C), or (G+C)/(A+T+G+C), are completely different ratios unrelated to Chargaff's rules. These are roughly constant across individuals for a given species and can be used to distinguish different species' genomes in an approximate manner.
I am not your professor, but it is possible in writing what they wrote, the professor was referring to whole-genome GC content within members of a species, in which case the professor is correct. If the professor was discussing Chargaff's rules, then the professor was wrong, but probably just made a mistake in writing: these things happen.