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It appears that the author has chosen to apply an additional factor in the definition of RPO by stating that a backup cycle of one complete backup every 24 hours results in an RPO of at least 48 hours (apparently because of an assumption that, in addition to a disaster occurring at 23.9 hrs, the most recent completed offsite backup may have an error. While that is a laudable concern it is not part of the accepted definition of RPO, and the author's assumption of one bad tape backup is no more or less defendable than an assumption of 0 or 2 bad backups. The standard to be imposed on the reliability of the backups is a separate issue and is not part of the definition of RPO.
I suggest that the author move the impact of backup system errors into a separate paragraph, as it is a useful factor to consider as part of the broader picture. Similarly, the mention of the risk of a single error affecting more bytes of data as media densities increase should be in such a separate section, perhaps titled Additional Factors Impacting Recovery.