The earlier version incorrectly treated sitting after every 2 rak'ahs in every voluntary prayer as an absolute validity condition.
The source itself says that if someone performs more than 2 voluntary rak'ahs and sits only at the end, the prayer is still valid by juristic preference because it has become one continuous prayer whose required final sitting exists.
So:
• sitting after every 2 rak'ahs is the normal and proper structure;
• omitting the middle sitting departs from that structure;
• but the whole voluntary prayer is not automatically invalid in every such case.