My charitable guess is that they lump the compute expense for free users as "Marketing".

My less than charitable guess is that the leaked financials is a piece of fiction intended to make inference not look unprofitable.

I see what you mean. If they are using it for customer acquisition, it could fall under sales and marketing.

What about retention though? Or upselling to higher tiers?

I would question where they decided to draw the line… because it’s a grey area.