No. When you send to a Bitcoin address, you actually lock that Bitcoin to a script. They're saying in Liana, you can lock Bitcoin to a single sig and multi-sig script. The single sig starts off invalid because it has a timelock applied to it in the script.

So it's not quite accurate to say it degrades into a single sig, because the multi-sig path is still valid, its just that you can also spend this single-sig path after the timelock.