MacInnes, who took UK gold at 100m and was European Under-23 champion at both distances last year, was the first – but not the last – to post an Olympic qualifying time at the London Aquatics Centre event.
“That’s insane to hear,” she said. “To be the first one to go under the time and in my event is really special.
“It’s ever athlete’s dream to go to the Olympics and I’m no different. I’ll just re-shift my focus to that and see how well I can do out there.”
MacInnes said there is a friendly rivalry between herself, Stephens and Emily Large, who finished third.
“I’ve got a lot of respect for Laura and Emily and, regardless of her being a world champion, it was really exciting to beat her,” she said. “We all get on really well.
“I knew I was catching up in the last 50 but didn’t know how much Laura had left. It was just about making sure I had a fast finish and kept building, building to the last.”