Will sp_flush_log also cause log records of uncommitted transactions to be written to disk?

It might, as a side-effect.

Any log blocks flushed to harden delayed durability transactions might also contain log records for uncommitted transactions.

This doesn't affect the state of those transactions, just whether their log records are in memory or on disk. The effect of any uncommitted transactions would be undone at recovery time as usual.

Will sp_flush_log also cause log records of uncommitted transactions to be written to disk?

It might, as a side-effect.

Any log blocks flushed to harden delayed durability transactions might also contain log records for uncommitted transactions.

This doesn't affect the state of those transactions, just whether their log records are in memory or on disk. The effect of any uncommitted transactions would be undone at recovery time as usual.

See The SQL Server Transaction Log, Part 2: Log Architecture by Paul Randal.