I fully disagree with that "guide", and agree with the book that
@Morbid Sea Cow posted. Maybe it doesn't technically meet the definition because it's not sourced from a poppy, but at that point it's just mincing words, because the chemicals act exactly like opiates in your brain.
Kratom and Other Mitragynines: The Chemistry and Pharmacology of Opioids from a Non-Opium Source.
The definitions you're looking for start on page 10. I can't copy paste from the document.
Opioids such as morphine, codeine, and oxycodone are extracts or analogs isolated from a single source: the opium poppy. For a long time, it was believed to be nature's only source of opioids. But it now appears that biological diversity has evolved an alternative source of opioid...
books.google.com
I've noticed this pattern of addiction denial from kratom users. I got banned from the Reddit kratom sub for suggesting to a mod that she was an addict because she was drinking 20 plus grams a day for years. But she could "quit when she wants with no withdrawal" yeah, sure you can. It was "chemical dependency" not addiction

. On Reddit, they completely deny that kratom has any withdrawal symptoms or addiction potential. Let me tell you, as an opiate addict for more than 10 years, kratom is EXACTLY THE SAME THING. It's addictive, even if you just try to tough it out and claim it's not withdrawal like many do. (A reddit person was very obviously kratom sick from months of heavy use, and the entire group concluded it HAD to have been the flu, because kratom "doesn't" cause withdraw.) Just because the withdrawals don't put you in a bed or hospital, it doesn't mean we're not addicted.
To everyone who reads the "it's not addictive" comments and thinks it won't happen to you, you're in for a rude surprise. Daily kratom use will cause physical dependence and withdraw if enough is used for long enough, like ANY other opiates.
For some reason, some people find it incredibly difficult to admit they are addicts, and will jump through incredible hoops to try to prove it to you.