They are portable in Europe (I said that in the second part of my comment), but it seemed like they weren’t in the US, because otherwise, the post is a bit strange. The author just tried every number from his area code to find the owner, when it is possible/likely that the owner has a completely different area code. That’s why I thought it might be that the current number always reflects the actual area code.
I have an old utah number that I've taken around to several states and I always get fun conversations when people ask what I did there.
Also, 03 is guaranteed to cost the same as 02 but has no specific geographic link, so it's often used for helplines, customer service, that sort of thing, and for people who want to have a landline (or multiple landlines) but do not want to reveal their location.
04, 05 and 06 are part of that roped off space we reserved in expectation of a need that's now unlikely to ever materialise, oh well. In the era when it was conceived people thought "Internet shopping" was a ludicrous idea and still imagined "Video calls" would be a thing you'd do as a telephone call somewhow - so what did they know.
07 as you said is where non-geographic mobiles live, as well as some other services at similar price points
08 is "free" or sometimes revenue sharing prefixes and the 09 prefix is where "premium" services live, you know "Chat live now to singles in your area".
In the USA, the number reflects the area it was issued. So my phone number matches suburban DC. If I move to California, everybody would know I lived in DC in the past. Kind of odd, I guess, but is a leftover from when cell homes didn’t exist and then when numbers weren’t portable across providers.