a positive offset on the triangle wave will shift the triangle with respect to the input, and will cause an offset after the output signal is filtered. if such an offset exists, it can be countered by a similar offset applied to the input. if the modulator is being run from a single supply and not a bipolar supply, both inputs will have an offset of 1/2 Vcc to begin with. just as long as the offset on the triangle, and on the input signal are the same, there will be no offset at the filtered output (except the 1/2 Vcc level because of single supply operation). so in the case of single supply operation, 1/2 Vcc is "zero", and any deviation from that is an offset.