That is wrong. Southridge is in 3A now as you said due to getting the 6 points in 2017 and 2018. Here is the part of the rule that explains what is required to keep them in 3A.
d. If a school which has participated in a higher enrollment class during the previous reclassification period achieves a tournament series success point value of Two (2), Three (3), Four (4) or Five (5) points in a specific sport, such school shall remain in the same enrollment class in that sport for the next reclassification period.
Southridge did not win the sectional last year so they received 0 points. A regional championship this year would give them 2 points. That would meet the requirements to stay bumped up to 3A.