Well, they are making a somewhat consistent $30-35M a year (check the funding chart), and a $2.5M monthly burn rate is indeed sustainable at $30M (how the hell they keep raising that is a different question altogether) and a $3M burn rate is just barely below that they have made these last three years (closer to $35M).
At $2.5M and 450 employees that amounts to $5.5k per employee per month, which, around here, is indeed quite in the ballpark of developer cost (including overhead) and even somewhat high (it would assume experienced although not top level devs, juniors would be cheaper). Their operation being distributed around different places makes it harder to justify (I would think the expected US cost per employee is higher), but we also don’t know how many of those are (low or unpaid) intern positions.
I mean, other than the annual funding being so mysteriously high, the numbers could make some sense (there’s much data we don’t know to give more that a guess).