That seems like a silly choice, since most diesel trains are diesel-electric... which is a design that can easily be modified to run dual-mode.

They probably mean diesel despite the picture chosen not making it obvious.

??? They said diesel.

Yes. They’re saying that it’s an example of a diesel locomotive, despite the picture suggesting it may be electric only.

I translated the polish language page it is on and it sounds like it is diesel with a mechanical drive.

If I understand correctly those are just somewhat cheaper per unit and some of them do run on lines that are not electrified. So it happens that a diesel train runs entirely (or often partially) on electrified lines like in the photo I gave.

I said "diesel" as a mental shortcut for diesel and diesel-electric, but certainly not for electro-diesel.