Re: "calculations in an office are not done on a graphics card"
In other words, it's a covert channel - a means for an attacker to exfiltrate information - not a side channel. Side channels inadvertently leak sensitive data; covert channels are deliberately employed to expose it.
I don't think it's impractical for some specialized applications. The researchers say the signal penetrated a wall well and the carrier was detectable at ~15m, though they couldn't recover a usable signal at that distance. But someone who can plant a receiver in a closet next to an office, for example, might make use of this.
The same is true for plenty of other EMF channels, of course. The researcher's blog post makes the obligatory reference to TEMPEST right in the title - though TEMPEST focused on side channels, not covert channels.