1. How come it's so low? If Tech-Guard is equivalent of Imperial Guard, then Avernite Tech-Guard should be Helltrooper equivalent, with Tech-Guard grenadiers being the equivalent of IG grenadiers, so Avernite Tech-Guard grenadiers are Helguard equivalent. Skitarii are above them thanks to their extensive cybernetics and wargear upgrades, except with us the difference is even more pronounced thanks to our Dark Age tech. Myrmidons are supposed to be even higher than that, with an even bigger increase in the difference pronouncement caused by Dark Age tech.
2. Who would win in a fight, Avernite Helltroopers or Callamus Tech-Guard (from Deus ex Mechanicus)? Avernites are from Avernus and have Dark Age tech. Callamites have A-rank training (B-rank is stormtrooper level), A-rank equipment, Mechanicus implants, and biological engineering.