AC requires HF to keep the arc lit when the voltage goes to "0" as the polarity changes. This is the rule on transformer [sine wave] machines, Yours is one of these.
Some inverters [square wave] are supposed to be able to work on AC without HF, but these have a really fast transition from full voltage on one polarity to full voltage on the other, so the arc stays lit.
Having said all that, I never really tried it. I have used an old sine wave TIG machine that did TIG weld on AC, but the HF was not working wwell enough for arc starting with a cold electrode.