Didn't really capture concept well/misleading.

The reason you can tell if a substance you're analyzing contains bromine or chlorine is because of the characteristic ratio of M:M+2 peaks (specifically for a molecule containing a single bromine or chlorine). Bromine and chlorine exists as a mixture of isotopes with two substantial relative abundances:

Chlorine:
~(25% 37Cl and 75% 35Cl)

Explanation of Mass Spec including relative abundance of chlorine and bromine.
https://www.khanacademy.org/science/ap-chemistry-beta/x2eef969c74e0d802:atomic-structure-and-properties/x2eef969c74e0d802:mass-spectrometry-of-elements/a/isotopes-and-mass-spectrometry

Another explanation of mass spec (not directly relevant to Cl and Br but provides further general evidence for suggestion)
https://www.khanacademy.org/test-prep/mcat/physical-processes/atomic-nucleus/v/mass-spectrometer

Not Primary MCAT Source but directly discusses this topic for further support.

https://chem.libretexts.org/Courses/Providence_College/CHM_331_Advanced_Analytical_Chemistry_1/16%3A_Molecular_Mass_Spectrometry/16.09%3A_Organic_Compounds_Containing_Halogen_Atoms

Need to add specifically that this is the mass spectrum of a molecule containing a single chlorine or else this ratio is not true. For example, For molecules containing two chlorines .
Given: Chlorine: ~(25% 37Cl and 75% 35Cl)
35 + 35 = M
35 + 37 = M + 2
37 + 37 = M+4
(M+, M+2 and M+4) peak heights will be in the ratio of 9:6:1 when the compound contains 2 chlorine atoms. SO this no longer applies. I think having to specify that it contains one chlorine helps drive home this ratio/point.