Bunker fuel isn't a fuel. It's a name for a different set of fuels that ships use. Among those is Heavy Fuel Oil (which is effectively banned now), Marine Fuel Oil (what you are talking about, a low sulphur and slightly lighter variant of HFO), and ... Marine Diesel Oil (actual diesel which all container ships need to run on when they get close to port because you haven't been allowed to burn HFO or MFO anywhere near populated areas for a long time now.
And while it certainly used to be sludge that is left over, these days there are increasingly tighter restrictions on what MFO can contain, and it is very much starting to look more and more like diesel in composition (the last decade or so many refineries have focused on equipment upgrades precisely because they would no longer be able to sell their sludge to the shipping industry).
It's not incorrect to call these ships diesel powered. In fact for some of their journey the only difference between what they put in their engines and what you put in yours is the colour (colour denoting that the fuel was exempt from local taxes and may only be used for marine purposes)