ABT has back up performers for the lead roles waiting in the audience or backstage in the event the lead performers cannot continue. The show must go on. It is part of their assigned schedule from ABT. So Hee Seo did not coincidentally happen to be nearby and available to perform. It was part of her job as the understudy for O/O on Monday evening. She was very lucky that Gomes was her partner. In lesser hands the evening very well might have been a disaster.
The theory that Hallberg is replaced because he's too tired after his extra Giselle doesn't seem plausible. Why would they give the performance to Gomes, someone who just performed on Monday and is now - justifiably - even more tired than Hallberg.
Of course ABT has back-up performers and Seo would have been in the theatre, but, she would not have been sitting backstage in full stage make-up for SL with her hair done and have been already warmed up ready to jump onstage.
Who knows the reason for the Hallberg replacement tomorrow night, but Gomes has already proved himself to be more capable than any other male dancer in the company so I can definitely see him able to dance another SL so soon but not Hallberg.