To rephrase:
You can only provide a somatic component with a hand that is holding a spellcasting focus if the spell also has a material component. (And that component is neither costly nor expended.)
Which is weird as all get out. I see where it's coming from, but I'm not at all sure I like it.
If that were the original intent of the rules, they did a poor job of defining things in the books. That rule essentially means that there are two different kinds of somatic components. You have your free-hand only somatic components, and then you have your somatic components that take place when you cast a spell with a material component.
Since there are two types of somatic components, they should have just rolled one into the M component and straight up described M and S components as mutually exclusive. No spell could have both. Then they say that you can use a spellcasting focus to substitute for non-costly, non-expended M components.
That's exactly how Sage Advice is effectively saying that the rules work, which means either the rules were written extremely poorly or he is wrong about design intent. (Or he is only answering RAW as he reads it and ignoring intent.)
These rules also make it a liability to use a spellcasting focus unless the only spells you ever cast require material components (but not costly nor consumed ones), because you now will have to worry about switching that focus in and out of your hand all the time, which is a headache for both players and DMs to keep track of.
Honestly, it is a heck of a lot easier to just allow you to produce somatic components with a spellcasting focus in hand period.