I'm guessing the 'scalable' refers to the ability to use 2 cards instead of 1. If motherboards can support up to 32/48/64 PCIe Lanes (none of which appear likely currently) then it is possible that additional PCIe x16 slots could be added to support multi-GPU rendering with more than 2 cards. It may also be possible to use PCIe x8 slots instead of x16 slots, since that's all SLI supports anyhow, but same situation, just not as high requirements on PCIe lanes (theoretically, with 36-40 PCIe lanes you could run 4 graphics cards at x8 and still have room for other components. But that's still talking almost twice what's available now.). I suppose they'd also have to redesign the SLI connector to support in excess of 2 cards as well, but I don't imagine that bit would be too hard. The trick is increasing the number of PCIe lanes to the connectors. Don't expect to be using 3-4 graphics cards anytime soon, though.
Also keep in mind that all of what I've said here is based on what I know of SLI and motherboards, which is to say, not that much. Take my info with a grain of salt, I don't know it all. But those are my best guesses from how I understand the information.
You also may want to check out a thread about a video card that uses SLI in the card and does not require two cards to run in SLI mode, but you'd have to run a search since I don't know how to link and I'm too lazy to look it up myself =p
Discussion Zen 5 Speculation (EPYC Turin and Strix Point/Granite Ridge - Ryzen 9000)
Discussion Intel current and future Lakes & Rapids thread
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