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.

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.