This is true yes but often over emphasised ITO their exclusive use of the Mausers. Remember that many Boers were not wealthy and when joining Kommandos during the second Boer War they were encouraged to bring their own rifles (some even used shotguns - as evidenced from British surgeons reports). Some even had the money for high end 95 Plezier Mausers, other used what they had. There were many Mausers but many Boers used Lee Metfords. Boers shot whatever they could afford or get.
If you look at old Boer War photos you will see Boers with all kinds of Mausers from the .43" 71, 88 commission rifle right on through the 95, Lee Metfords, Martinis, Krag-Jorgsens, Snyders and I am sure the Lebel and Fusil Gras must have filtered down from the French colonies.
Lee Enfields became popular once they arrived even though they were the rifle of the 'enemy'.
Yes, at the margins they were superior to the Brits (they had local knowledge and were better shots) but the scorched earth tactics of Kitchener and the shear numbers of the British troop/artilery surge along with a tactical error or two by the Boers ended it officially for them.