Literally as written, this sounds like a medical emergency.

From the article: "It had its right turn indicator on, but it drifted slowly to the left, into my lane, before slowly going back into its own lane."

For this to have happened, the vehicle could not have had autosteer engaged. At most, it would have had regular traffic aware cruise control on, which would not have been monitoring for wheel input at all anyway. This strongly implies to me that the operator was at least operating the vehicle intentionally to a significant degree.

In any event, calling 911 is an appropriate response to an unresponsive driver.