What does "serious" mean in this context?

In this context it would presumably mean not casually deleting your customers' data.

When people say a company is unserious they almost always mean that they're not thinking about, and taking seriously, the work that their customers do. This seems like a classic example of that.

producing an os with a focus on utility to end user would be a start. Even just something that makes some form of effort to give users a return on investment on their hardware. Right now, you unluck massive headroom, battery life, and stability, on a laptop by simple putting linux on it. All that despite windows enjoying better all around driver support from manufacturers.

A serious company would never have allowed its core product to turn into the clown show it currently is, yet here we are, a massive corporation pushes an OS that feels vibe coded, and is largly propped up by what little residual community goodwill/ignorance exsists.

Dependable.

It's just an internet meme phrase. "X is not a serious Y" just means that X is a joke.

It's a dumb phrase and overused in contexts like this.