That implies that the previous lack of modularization was a practical decision, not a political or philosophical one. Implementation of the GCC plugins system started in 2008. Can you imagine the GCC team completely changing their philosophical stance almost overnight (Clang's first release was in mid 2007), just to get more users? It would be as if they'd relicensed to proprietary software to better compete with VC++.
Previously GCC's primary competitor was ICC, and they were compared based on the quality of their output. Now GCC's primary competitor is Clang, and they compete on how user-friendly their interface is. Modularization is a user interface improvement, just like better error messages or more comprehensive warnings.
The claim I've seen repeatedly is that this is precisely because they must now compete with Clang/etc.