It is not with the pipe per se that you have a problem with, as greek characters are well displayed in title and other fields, but with the deprecated fetch page module.
Use the XPath fetch page module instead (let's call it XFM in the following), it works. here I took the link of the first post of the feed (at the time I'm writing those lines at least) and fetched the content you fetch with the fetch page module. Greek characters are well displayed.
However, be careful with XFM. You'll see that I put 2 of the XFMs.
In the first one I fetch each <p>...</p> sub-items, but as a general rule, there are some mark-ups in the middle of the text. In this case, Pipes casts the beginning of the text in the content sub-item, and the following mark-ups in the corresponding sub-items. Then, it casts the rest of the text to the same content sub-item, more or less erasing the beginning of the text (some weird stuff happen here; and I guess the same bug exist for similar mark ups in the text).
This is a known issue of the XFM (hello Paul :) ).
The only work-around I found is to emit the whole content as a string, this is the second XFM. Then you clean it up as you did with the content from the fetch page module.