They are most likely only looking at the RAM used but the 'Discord' process, which is just the native launcher code and OS hooks (on MacOS here, may be different on other OSes). The electron processes are called 'Discord Helper' and will be using quite a bit more RAM, though I still wouldn't consider it 'heavy'. The same process structure is true of Slack as well (maybe all electron apps, I'm not familiar enough to say).

Both Slack / Discord use around 400MB including all processes on my machine, tho it certainly varies with what is on screen. In comparison to the gmail tab I have open in Safari that is using 1GB, 400MB isn't bad.