There doesn't seem to be any restriction that the word needs to consist only of lowercase letters, so words of any case are included. Since the example code specifically shows the example words (TEA, EAT, ATE) in uppercase, I elected to uppercase the found words.
As the specs keep changing, this version will accept ANY text file as its dictionary and accepts parameters to configure the minimum number of characters in a word to consider and whether to allow mono-character words.
Defaults to unixdict.txt, minimum 3 characters and mono-character 'words' disallowed. Feed a file name to use a different word list, an integer to --min-chars and/or a truthy value to --mono to allow mono-chars.