It was a bit of a hyperbole, I have no idea about the exact amount.
Let’s say you charge your 2000mAh battery every day and your PSU is 10% more efficient than your charger (the difference is most likely not even this big).
2Ah × 5V x 356d= 3.56kwh
3.56kwh × 0.1 = 356Wh
356Wh would be the difference per year, that’s about 12ct per year.
Now estimating the power usage for fediverse messages is very hard to do since it depends on a lot of different factors (your device, cellular or WiFi data, amount of hops needed to reach you, general state of your nearby network, your instances infrastructure).
The only even remotely similar thing I could find was emails with pictures producing about 20-40g CO2, which only slightly increases with more recipients, and Reddit usage comes at about 2.5g per minute. Comparing these two numbers just shows that all estimates done are pretty much useless for us since we have no idea how they are done.
But if we go with a low estimate of 0.1g (slightly above SMS and somewhere around spammail level) per user seeing it and a few hundred to a thousand users seeing this even if they just scroll past, we reach the CO2 equivalent of 1kWh pretty fast without even talking about long term storage and future indexing. Not to mention that comments produce something too since they need to be federated, albeit not so much as the post itself.
So while 10 years was a bit much, 2-3 years would be very much in the realm of possibilities, but no one knows or can even properly estimate the actual numbers.
Switch to where? Everything that’s not just a different youtube frontend is either shit or doesn’t pay the creators. Federated FOSS sites aren’t an option either cause once an influx of users outside the tech bubble happens, the server capacity will hit ground bottom.