Note that the excerpt for microtransactions currently contains the text "also referred to as... in-app purchasing."
I think microtransactions and IAP are two different concepts:
- microtransactions deals with the business model
- iap deals with an implementation of the business model.
You can very well create a game that has no in-app-purchase but has microtransactions (what if you need to go on the website of the MMO to do the transaction), and you can have in-app purchase without it being a microtransation (you can buy the full game).
They should not be synonyms, and the text of microtransactions should be modified to remove the part about in-app billing.
In-App Purchases (IAP) should be a synonym of microtransactions as there is no way to do one without doing the other (although a $100 price tag might not be considered micro the code, apis, and so on involved are identical).
The only case I can see for having them be separate is is iap is referring to a specific microtransaction API, in which case it should be renamed (assuming we want the tag) to accurately reflect that, e.g. google-play-iap or unity-iap.