I had a near similar experience with Bing ads. Since you've never received any external traffic. I'd say the external traffic was somehow triggered by the ad. Probably a combination of crawlers, facebook bots and legitimate clicks. With a tracker you would have been able to tell how much of the external traffic were crawlers and bots, this would have also given you a deeper insight on the trafffic you received. The article you used at the end the CTA was mind cafe. A very important metric would have been your Opt-in conversion rate (CR). This can be a maker or breaker of a campaign. It can tell the quality of your targeted audience and also the quality of your writing and how well your copy converts. May be mind cafe had a small spike in their opt-ins that week 😂. Next time consider having a CTA that you can measure Conversion Rate and a tracker as well if possible. Later on after monetizing your list you can equate a value in dollars for each email you collect, this can be a good way to predetermine your ROI.