I think it very much depends on whether or not you have any bargaining power or leeway in the company. As he says further down, if they won't allow you to do ANY side projects (or at least severely restrict you), you counter by demanding for more pay or other compensation for your "loyalty" (for a lack of better word) to focusing solely on the company's products and code.

Alternatively, argue that letting you work on solo projects will be beneficial to the employer; Risk-free testing of new frameworks and languages that leads to an experienced developer.

For some very big companies (I've worked at one myself) they seem to have a zero tolerance policy of outside work and you might be SOL in that case - I guess you have to ask yourself whether it's worth it then.