Aside: At the end of the article they have a "Bonus: Speeding things up" section where they automate adding 300~ coupons via 300 HTTP connections in 5 seconds (instead of 60~ seconds).
In my opinion if you're going to automate stuff like this, you should do so with the goal of minimizing disruption (and frankly, detection). They run the script automatically at midnight in the background via cron, so why was going slower problematic? 300 requests in a span of 5 seconds seems much more likely to trigger an IDS[0], get flagged as unusual in the logs, or similar than 300 over 60 seconds. Particularly at midnight.
I'd be trying to look at human as possible and not set off automated security systems. Heck you could add a randomized delay (e.g. 1~2 seconds) between requests and it would still be completed inside of 10 minutes. Plus then nobody can reasonably accuse you of trying to "DoS" them/violate the CFAA.
[0] https://en.wikipedia.org/wiki/Intrusion_detection_system