I suppose so, although I can't say I understand enough about how much cation ordering or structural distortion from calcite is necessary to define something as a member of the dolomite group. Am waiting for a mineralogist to chime in.   ;)) 

There are other examples of such "anomalous" Ca-dominant "kutnohorites" in the literature on Mn/Ca carbonates, especially from metamorphosed Mn mineralisations/deposits.

PXRD can well distinguish between a calcite-type carbonate solid solution and a cation-ordered dolomite-type carbonate solid solution, but there is a gradual change between them, with broadening of reflections. There is a lot of literature on this.

In principle it must be considered a new, Mn-stabilised CaCa-member of the dolomite group.

But referencing an old discussion where I assert that end-member formulas can't have commas in them, I suspect we may end up with something like:

calcite: from CaCO₃ (end-member) to (Ca_1-xMn_x)CO₃, where x represents the maximum amount of Mn that calcite can take in at ambient conditions.

solvus: this will be the 2-phase region with separate co-existing (Ca_1-xMn_x)CO₃ (calcite; see above) and Ca(Ca_1-yMn_y)[CO₃]₂ (new mineral; see below).

new mineral: from Ca(Ca_1-yMn_y)[CO₃]₂ (where y is some value less than 0.5 and is the minimum value required to reduce the symmetry from 3 2/m to 3; the formula using this limiting value would be the end-member formula of the new mineral, eliminating the need for a comma) to Ca(Ca_0.5Mn_0.5)[CO₃]₂, which is the boundary where any more Mn-rich compositions would be kutnohorite.

kutnohorite: from the Ca(Ca_0.5Mn_0.5)[CO₃]₂ boundary composition through the end-member composition CaMn[CO₃]₂, and then onwards to the most Mn-rich composition on the edge of the solvus with rhodochrosite.

If the "y" in the above formula turned out to be 0.40, for example, then the nominal end-member formula would be Ca(Ca_0.60Mn_0.40)[CO₃]₂, because this would be its limiting composition along the CaCO₃-CaMn[CO₃]₂ binary join. But the limiting compositions of natural samples with additional Fe²⁺ or Mg could be expected vary a bit from that, potentially on either side of the pure binary limiting composition. So the addition of some Fe²⁺ or Mg might come only at the expense of Mn, or it could also somewhat increase or decrease the Ca_0.60. With how long and extensively the rhombohedral carbonates have been studied, I'd actually be surprised if someone hadn't already figured all this out. But maybe people have explored the various solid solution limits without having ever bothered to check the symmetry?

But in any case, one would still have a definitive "end-member" formula for writing simplified reactions and for referencing derived thermodynamic data.

Normally with carbonates we normalise to one or two cations as we don't analyse for C etc.