It was closed-source for the entirety of Android 3.x “Honeycomb”, the tablet-only Android version that introduced Holo on the Motorola XOOM etc. Source code for 3.x was retroactively made available when 4.0 resumed its open-sourceness:
> This release includes the full history of the Android source code tree, which naturally includes all the source code for the Honeycomb releases. However, since Honeycomb was a little incomplete, we want everyone to focus on Ice Cream Sandwich. So, we haven't created any tags that correspond to the Honeycomb releases (even though the changes are present in the history.)
https://groups.google.com/g/android-building/c/T4XZJCZnqF8
https://hackernews.hn/item?id=3235926
> This release includes the full history of the Android source code tree, which naturally includes all the source code for the Honeycomb releases. However, since Honeycomb was a little incomplete, we want everyone to focus on Ice Cream Sandwich. So, we haven't created any tags that correspond to the Honeycomb releases (even though the changes are present in the history.)