I thought that Olney campaigned against hard Brexit, not against Brexit. Certainly, in her victory speech that is what she said.
Quote: "The people of Richmond Park and North Kingston have sent a shockwave through this Conservative Brexit government, and our message is clear - we do not want a hard Brexit."
That seems fairly clear, and nobody can say it's undemocratic, since the referendum didn't specify hard or soft.
It's difficult to say what it means, since it's obviously a strong Remain area. I suppose it might make May think twice about an election.