Well, despite the fact that Geany never had a 1.0, its "interesting" versioning system having gone from 0.22 to 1.23, we do attempt to keep the ABI and API as stable as reasonably possible.
In general the ABI changes as little as we can manage, the ABI number must match exactly between Geany and the plugin, so an ABI change kills all plugins until they are re-compiled. But sometimes it is absolutely necessary. Removing a feature or significantly changing the semantics of an existing feature require an ABI change since existing plugins may no longer work with the modified version of Geany.
The API identifying number is increased whenever anything is added to the API so plugins can test if the feature is available. The API number required by a plugin needs only to be lower than the API Geany provides, so an increase in API number without a change in ABI will not stop plugins that need a lower number from working.
These rules seem to work reasonably well, and are likely to continue into the future.
All of which is a long way of saying that whilst the exact text (referring to 1.0) is wrong, the general sentiment still applies.