There's an error in the text. "Tulishakula" means "We finished eating", if you want to say "We have already eaten" you should say "Tumeshakula".
in case someone is reading this later, yes, this is correct, and is now accepted
Is this "had eaten" or "had already eaten"? Is "already" an embellishment or a real part of the meaning?
this would be 'had already eaten', with the infix '-sha-' representing 'already', whereas 'we had eaten fruit', would be 'tulikuwa tumekula matunda'. However, past participle is not very commonly used in Swahili without '-sha-'