Maybe preventing Brexit was exactly the purpose based on "advice" from others. Why would someone push so hard for such a vital election days before the Brexit negotiations? She pushes a fast election based on polls? They had a majority already with 330 seats, so I'm sure there were some anti-Brexits were just hoping she'd fail. It's possible, she's been played by some who wanted her to lose seats.
What makes no sense about people saying Brexit is at risk, is that the far left party was also supportive of and assured the voters that Brexit will happen, one of the reasons some say they made such inroads, since it was no longer an issue to debate. The two parties who overwhelmingly won the vote were both on course for a quick and effective separation from Brexit, so this doesn't change much. She will form a minority with the help of DUP and the show will go on.