Per capita sqft of office space is much lower than everyone having their own room (even a small 9x9 one). In other words, office heating/cooling is amortized across a large population.

I'm pretty sure there are lower hanging fruits than trying to pack as many people as you can in one space to optimize carbon footprint. I find this argument to be a far fetched way to guilt someone who works remotely at best. What's the next step, why have homes or apartments that generate excess carbon emissions when you can pitch a tent in the open space inside office buildings?

There is a big spectrum of possibilities between "everyone having a typical American suburban home with an extra room for their office" and "pitch a tent in the open space". Walkable dense cities is one such example. It is no surprise that NYC's carbon footprint is 11 MTCO2 per person while the number for the overall US is ~14.5 MTCO2. Or, NYC gives you almost one-fourth reduction in the carbon footprint per capita.