(the Mac Mini and the iMac count as laptops as they use laptop components)
No they don't. The iMac and the Mac Mini are desktop computers (or computers in the fashion of desktops), because that's their use case. Typically, when one buys an iMac or a Mac Mini, the competing products they're looking at are the HP, Dell, etc. DESKTOP lines. Likewise, someone buying a Macbook will look at other Laptops.
Perhaps the programmers they have working on the Linux version are better than the programmers they got to do the Mac version? Perhaps they know the system better (e.g. the programmers writing the Mac version are Windows programmers who got reassigned, vs. the Linux version where they specifically hired Linux devs)? Perhaps Valve learned some stuff when porting Steam to Mac that they couldn't apply to the Mac version (because it was too far along), but had the advantage of applying to the Linux version? Perhaps they have another reason to make the Linux version particularly better (rumoured Steam console)?
Point is, there are lot of reasons a Linux port might be better than a Mac port.
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Where there's a will, there's a relative.