The author is actually preferentially sending samples towards "attractors" [0] for lights and dielectrics. Doing so massively reduces variance. My favorite example is Figure 5 in this old paper [1] (I've never actually read Pete's ray tracing in a weekend, so I don't know if this is in there).
The most common variance reduction technique though would be to do direct lighting (aka "next event estimation" if you want to be overly pedantic). Your lighting in that image looks like it's just a "Hi, I happen to be emitting tons of energy" sphere in space, which will cause a lot of noise. Alternatively, if you are sampling it, you are likely not sampling the sphere as well as you could. You'll want to sample the sphere following the setup in Figure 2 in Pete's "Direct Lighting" paper [2] (which as a reminder, is currently accessible at the ACM during Covid-19).
[0] https://github.com/Dalamar42/rayt/blob/fc57fa4afc080a578e21e...