Spotlight gets backup information from DMV msdb.dbo.backupset:
select distinct database_name,
MAX(CASE WHEN [type] = N'D' THEN backup_finish_date END) [FullBackup],
MAX(CASE WHEN [type] = N'I' THEN backup_finish_date END) [DifferentialBackup],
MAX(CASE WHEN [type] = N'L' THEN backup_finish_date END) [LogBackup]
from msdb.dbo.backupset
where db_id(database_name) is not null
group by database_name
Any 3rd party backup application must write into msdb.dbo.backupset for Spotlight to reconginze the backup.