This is the 14757th most frequent Polish word.
płytki
Shallow.
Here, 'płytkie' ('shallow') is an adjective describing the depth of the interior of an object.
Ten kubek ma bardzo płytkie wnętrze.
This mug has a very shallow interior.
In this context, 'płytkie' ('superficial') metaphorically describes the depth or quality of arguments.
Jego argumenty były naprawdę płytkie.
His arguments were really superficial.
Here, 'płytki' ('shallow') is used to describe the physical depth of the pool.