Auditlogs from before setting up the export

Enabling a Diagnostic Setting today does not automatically backfill older resource logs into Log Analytics.

Diagnostic Settings begin routing logs after they are configured.

There is one important distinction for the Azure Activity Log.

Azure keeps Activity Log events natively for a limited retention period, so recent Activity Log entries may still be visible through the Azure portal even though they were never exported to Log Analytics.

However, configuring Log Analytics afterwards will not automatically import that historical Activity Log data into the workspace.

For long-term retention you should configure Diagnostic Settings in advance and route the required logs to:

Log Analytics

Storage Account

Event Hub

So the short answer is: No, enabling the export now will not retroactively populate old resource/audit logs in Log Analytics.