Information that I have come across:-
The 24 hours is the time to get the SAE report to the Sponsor performing SAE/SUSAR reception and evaluation. If the Sponsor has set up a system where that notification has to go to the CRO first, then they need a robust process to ensure that the CRO passes it on immediately to the sponsor to meet that requirement.

There could be a scenario where a Sponsor outsources almost everything (but obviously not the oversight of the CROs by the sponsor,- that always remains a sponsor responsibility). This would mean that the reception and evaluation of SAE/SUSARs will be performed by a legally contracted vendor/CRO. In that scenario it would be expected that the investigator report in 24 hours to the CRO – essentially the timeline is there to make sure that the information gets quickly to those that need to report it, and of course within the legislative timelines to the Regulatory Authorities