It is NOT Polish. The digraph “cz” comes from the old Czech orthography, which was taken over by Latin and from Latin by English (Czech, Czechia) to express pronunciation of /t͡ʃ/ (IPA key). In Czech “cz” was used until the beginning of 19th century, since the standardization of modern orthography has been substituted by the consonant “č” (however it was introduced already at the break of 14th and 15th century), usually denoting the voiceless postalveolar affricate consonant [t͡ʃ] like the English “ch” in the word chocolate. In Polish the “cz” digraph is still used, therefore, that etymological mistake about Polish origin survives.
P.S. It is necessary to mention that English Wikipedia wrongly says (and rejects correction /!!/), that “cz” in English came from Polish at the end of 18th century (without any source), which is a nonsense, because in Latin “cz” is documented already in 16th century (e.g. “Czechia” from 1569 in the Latin preface to the “Musica” of Jan Blahoslav) and this digraph in various forms (adjectives, nationality, name of the country) is very frequent in Latin texts in Baroque period. In addition, the first recorded use of a similar (however not correct) name is in English in the word “Czechians” by Peter Heylyn in 1625 in his “Mikrokosmos: A little description of the great world. Augmented and reuised.” (the second edition) on page 298, explicitly referencing Czech sources, including Jan Dubravius: . This clearly shows that the word “Czech” does not come from Polish in the 19th century, which is a myth.