First, Mary Michael Patterson is not an understudy, but an alternate. She has two scheduled performances a week. She is one of the main cast members.
Second, I’m a fan of diplomacy. And I definitely assume they celebrated because all principals were on. But in this case it seems to be a lot about Sierra and very little about logic. Main reason: “Phantom of the Opera” has celebrated rather prestigious milestones in the past with alternates and/or understudies on:
*When the Broadway production officially celebrated that it was entered into the Guinness Book of World Records in September 2012, it was Marni Raab (the alternate at the time) they photographed along with Hugh Panaro and Kyle Barisich.
*When the West End production celebrated its 10.000th performance in October 2010, with Andrew Lloyd Webber and Michael Crawford present, u/s Stephen John Davies played the Phantom. Because of illness, but still. They could have moved the event if it was vital for them to have then principal Scott Davies on.
*In 2004, when the Broadway production celebrated its 7,000th performance, alternate Christine Julie Hanson was on, along with principals Hugh Panaro and John Cudia.
*When the Broadway production celebrated its 21th anniversary in January 2009, the role of Christine was played by alternate Elizabeth Loyacano, along with principals Howard McGillin and Tim Martin Gleason.
I agree with you as far as them wanting to celebrate when the principals was on (despite tradition). But I think the REASON why they wanted to celebrate with the principals is that Sierra Boggess is one of the principals. Nothing more, nothing less.
I adore Sierra Boggess, don’t get me wrong. And they didn’t say in either of the updates that it WAS the 11,000 performance, just that they celebrated it that night. But I’m fairly sure they knew readers would perceive it as the cake being rolled in right after the 11,000th performance, with the cast depicted. The photos will forever be labeled as “from the 11,000th performance”, not “from the CELEBRATION of” said performance.
In short, Mary Michael Patterson performed the 11,000th performance on Broadway, but is not credited for it anywhere - very unlike previous tradition of celebrating the event the same night no matter the cast, crediting the actual cast that performed.