Use ln OutPoints not bitcoin ones in SpendableOutputDescriptors
Lightning OutPoints only have 16 bits to express the output index
instead of Bitcoin's 32 bits, implying that some outputs are
possibly not expressible as lightning OutPoints. However, such
OutPoints can never be hit within the lightning protocol, and must
be on-chain spam sent by a third party wishing to donate us money.
Still, in order to do so, the third party would need to fill nearly
an entire block with garbage, so this case should be relatively
safe.
A new comment in channelmonitor explains the reasoning a bit
further.