I'd guess that's $800 for services, perhaps $2K for rent, and depending on lifestyle around $700 for food, drinks etc in CDMX. There's a type of deal-hunting digital nomad that would laugh at $3.5K/mo being called "modest" but the author is correct: it's quite achievable for many types of remote workers or ex-pats.
It's common for higher-end apartments in Mexico City to have a room for a live-in maid (often on the roof). These quarters are very small, often just big enough for a cot when you account for water heaters or other equipment mounted in them. I don't think I could pay someone to live in such a small space. (Yes, I know it could be a step up from wherever they're living now.) To be 100% clear, I don't know that that's what this post is describing; also some people/cultures are raised with this arrangement and so aren't bothered by it.
You are right that many of the of older high-end apartments have these tiny rooms on the roof for the staff, but I found that repulsive. The place I got was in a new building and had a proper service suite inside with a decently-sized bedroom and nice bath.
It's common for higher-end apartments in Mexico City to have a room for a live-in maid (often on the roof). These quarters are very small, often just big enough for a cot when you account for water heaters or other equipment mounted in them. I don't think I could pay someone to live in such a small space. (Yes, I know it could be a step up from wherever they're living now.) To be 100% clear, I don't know that that's what this post is describing; also some people/cultures are raised with this arrangement and so aren't bothered by it.