Doesn't correctly capture concept the relative abundance of Br's isotopes without change. Misleading.

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 bromine or else this ratio is not true. I think having to specify that it contains one bromine helps drive home this ratio/point.

Given: Bromine has two stable isotopes—bromine-\[79\] and bromine-\[81\]. The relative abundance of the isotopes are \[50.70\%\] and \[49.30\%\]

Therefore for a molecule containing two bromines:

79 Br + 79 Br = M Peak
79 Br + 81 Br = M + 2 Peak
81 Br + 81 Br = M +4 Peak

79 Br + 81 Br = M + 2 Peak has highest abundance based on probability.

1:1 Ratio of 79 Br + 79 Br = M Peak to 81 Br + 81 Br = M +4 Peak

See: Isotopes Br2 to visualize this:
https://chem.libretexts.org/Courses/Purdue/Chem_26505%3A_Organic_Chemistry_I_(Lipton)/Chapter_5._Spectroscopy/5.2_Mass_Spectrometry