Is this offer available to existing SP customers ?
Graeme has answered that and that it was answered in the article.
There are a couple of issue to discuss. In general same supplier switches are not usually available via comparison websites. Considering the financing model that's not really surprising. An exception is British Gas which does normally allow same supplier switches via comparison websites.
A broader question perhaps is whether Scottish Power Online Energy 9 (without the £60 offer) is available to you as an existing customer. Again the Scottish Power website would have you believe not. I can say however that I was a Scottish Power online saver customer when they introduced Online Energy 5 which was also "not available" to existing customers. At the time I was free to switch away without penalty and "insisted" on being moved to Online Energy 5. After the exchange of a few emails the "landings team" switched me.
I suggest you take a hard nosed customer centric approach to this sort of "restrictive" practice. If you switch to a different supplier's tariff without a tie-in you would be free to switch again 28 days after transfer, including to the full choice of the then Scottish Power Saver tariffs.
This is pure speculation but I think this particular offer is a competitive but temporary offer while current market movements are considered. Given that the market is moving downwards (at the moment) a "hard-ball" approach is fairly low risk.