We didnt see her entire report to Celestia. I got the feeling like Twilight was, funnily enough, being a bit like Spike."Yeah, sure. I broke several eons-old spells cast by a mad wizard whom conquered an empire and made it to the heart. That wasn't anything special! The heart was the important part and in the end 'I' couldn't save it because I messed up. Spike had to step in."Obviously that's just a guess at what was running through her head, but given the tone and her chief worry of disappointing Celestia I dont think it's far off. It's not that Twilight just up and gave Spike all the credit (something he would never let her do anyway especially given all the work she actually did!), it's that she felt her own contributions mattered less because, in the end, she had to go against what Celestia had told her ("You and you alone must be the one to help Shining and Cadence"). So, to Twilight, Spike did save the day and all she did (from her own perspective) was disobey her teacher.Thankfully, Celestia pointed out to Twilight how she made the right call. You might remember the episode ends not only with Spike getting his own commemorative window but with everyone else singing a song about how great Twilight is. The fact that they built a statue of Spike in the empire and treat him like a prince doesn't even come up until the next season, plenty of time for everyone to learn all the details of what happened which nobody, least of all Spike, Twilight, and Celestia, would have had reason to hide.