"Huffman coding is unsuitable for text files"?
It's not unsuitable, it is just not optimal. That's because letters in human readable text are not independent, but quite strongly correlated. That correlation can be used to get huge savings. For example, the letters q and Q are almost always followed by u. Comma and period are almost always followed by a space character, and period space is almost always followed by an uppercase character. These correlations mean text can be compressed a lot more than by using Huffman coding. But Huffman coding does work and will you get some amount of compression.