That's not what the article said ...
"The swimming dinosaur had four limbs and it walked on its hind legs, which each had three toes," she said. "The tracks show how it became more buoyant as it waded into deeper water — the full footprints gradually become half-footprints and then only claw marks."
" then suddenly the claw impressions became deeper"
The running analogy and the wading analogy would have produced similar prints.
My point is that a two legged marine dinosaur would possible be feeding on land and in the water. Bipedal dinosaurs were after all the most efficient predators.
Think Godzilla.