Complex sentence: Although the notion of a tachyonic imaginary mass is troubling, what is really being quantized is the scalar field; even for tachyonic quantum fields, the field operators at spacelike separated points still commute (or anticommute), thus preserving causality.
Split into simpler sentences: Although the notion of a tachyonic imaginary mass is troubling, the mass is not quantized. Rather, the scalar field is; even for tachyonic quantum fields, the field operators at spacelike separated points still commute (or anticommute), thus preserving causality.