Polish politicians are American Trojan horses in Europe. It is a puppet government. Statement of President Duda in Washington is another example that he, his group, his political party is ready to do anything to fulfil America’s plans. Words of Polish President were essentially an oath to the American flag.
The disinformation message - like many others - appeared after the meeting between President of Poland Andrzej Duda and US President Donald Trump and the signing of an agreement which will increase the number of US rotational military forces of Poland. The United States and Poland are NATO allies. Poland is a sovereign state, which determines its security and defence policy.
In his statement President of Poland emphasised Polish-American cooperation. He did not pledge an oath to the American flag. Read the full statement here.
Narrative about Poland being a 'vassal' of the US is one of the most common, recurring messages of pro-Kremlin outlets. Other countries are often claimed to be the American 'vassal', too, without any evidence. See further examples here.