Which is to say that I find this claim highly unlikely. You're very lucky to have such immense latent productivity that was just waiting for the smartphone dam to burst.
I can't imagine why you wouldn't believe this, if you've ever had coworkers, and observed them spending half their time on their phone at work.
It's a distraction which hampers sustained attention and deeper thinking - as well as eats up actual minutes of time, some raw percentage of your work hours inevitably goes into garbage content on the phone instead.
I find the people who are skeptical about the idea that the phone frustrates doing deeper thinking, are often the ones who have never done it. This is why they don't see the value in it.
When I work at home or during evenings when I can focus for long duration my productivity explodes.
I have no issue believing the level of impact.
This is part of a bigger principle I've noticed where I can rely on sheer willpower, or I can simply make a small change to my environment (e.g. move the phone to another room) and that's a much more efficient way to achieve the same result.
It's partly about reducing the Temptation, and partly about setting a strong intention / setting a strong message to yourself. If you're serious about getting some real work done, then why are you even looking at your phone?
Eventually you can get to the point where wasting time in any way starts to feel gross and you catch yourself more and more, but for most people it takes a bit of recalibration to get there.