But does that room come at a higher carbon cost than commuting to the office (45mins each way(!) afair on average in the US) and having an office building that's entirely empty at night. That building needs maintenance, gardening, cleaning, ...

The gas engine in a car surely comes with a stellar carbon cost compared to heating another room in the house.

Then maybe work from home but in your car is the answer

So having a small room at home dedicated for work is bad? I don't really get it. How is office different, cooling/heating isn't needed there?

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.

Yeah but I already have the room and it is being cooled/heated whether I’m there or not. Driving 25 miles each way to the office on the other hand is something that I stopped doing completely once my work started letting me WFH full time.