[真]{ま}[逆]{ぎゃく} (not [真]{ま}[逆]{さか}, which is an interjection standardly used) is another 若者言葉, or an expression that is used mainly by young generation (with low education level) that makes me feel uncomfortable.
逆 is a polarity notion; Something can be either the original (元), or the opposite (逆). There is no intermediate values like "half-way opposite", hence no room for quantitative/qualitative notions like genuine (真) or fake/quasi (偽) to come into play. I have no idea what meaning 真 is adding to 逆. How is 真逆 different from 逆? How would you justify the addition of 真 to 逆?