Disagreed that they're only doing it to increase the 'active...

Disagreed that they're only doing it to increase the 'active users' metrics for no regard to return on investment. But right that it could skew the quarterly 'active traders' number (if US accounts fall into that metric). They won't be throwing away money to make 1 metric look better.

I see it as a way of cutting back on the over-priced marketing budget while market interest is low. And it could possibly result in a positive return (which the previous high marketing budget would not, at the current time).

I think the promotion is to accounts with <$500 account value.

Another possible confusion -

+617 active users last Q, $618k marketing (after cutting it from $1.6m peak) = $1000 per active (very expensive)
But I guess that's a net number. If the gross numbers were like -1400 attrition, +2017 additions, then = $306 per active (less bad)
I don't think they released that breakdown.
If my example was the case, then the marketing budget is less bad, but they'd have to focus on reducing attrition. Though it can only be dampened so much, as the economic factors are out of our control - people having less spare income to invest.