I guess you're right: traditional AV is required in this case.
From the R80.10 Mobile Access docs:
Anti-Virus and Anti-Malware Blade
Certain Anti-Virus settings configured for a Security Gateway in the Traditional Anti-Virus > Security Gateway > HTTP page of the Threat Prevention tab also apply to Mobile Access traffic. To activate traditional Anti-Virus protection, enable the Traditional Anti-Virus on the Security Gateway.
These settings apply to Mobile Access traffic when Traditional Anti-Virus is configured to scan traffic By File Direction:
- Incoming files arriving to - Inspects traffic that Mobile Access users upload to Mobile Access. (The drop-down menu is not relevant.)
- Outgoing files leaving - Inspects the traffic that Mobile Access users download from Mobile Access. (The drop-down menu is not relevant.)
- The Internal Files field is not relevant since Mobile Access uses an external interface.
- Exceptions are not supported.
If Traditional Anti-Virus is configured to scan traffic By IPs, all portal traffic is scanned according to the settings defined for the Mail, FTP and HTTP protocols in SmartDashboard.
I wouldn't call the Traditional AV "obsolete" as it just works differently than the newer Anti-Virus blade.
In fact, we recently replaced the Traditional AV engine as part of removing Kaspersky components from our product: How to disable and remove Kaspersky Lab components from Check Point Security Gateway