You're implying that atomic swaps apply only to scBTC and altcoin that belongs to the same chain, right?
I've gone through this thread and I find this post from Adam Back and it seems to confirm
that we can exchange coins across chains and, I dare to say, even BTC and scBTC.
You can do atomic swaps today.
You can do them between bitcoin and litecoin, it doesn't take a sidechain to do it.
But very few people use this feature.
There is a good description of the process in the blockstream side chain pdf in Appendix C.
http://www.blockstream.com/sidechains.pdf