A Czech native speaker told me that this phrase could also mean 'I am very clumsy' or 'I have messed up' as the name 'Matej' is referred to as a clumsy person. (equivalent to Kevin in German language).
Doesn't this translate as "I, I am Matej"? Doesn't jsem Matej mean "I am Matej"?
"I am Matěj" could be written without or without "já," and the meaning would be the same. In Czech it's very common for pronouns to be omitted when they represent the subject of the sentence.