I suppose that not having some standardized terminology, there will always be some confusion.
Then you add the complexity of international communication.
In the US at least, we differentiate dado from groove by grain direction, though otherwise they are the same. I believe the first time I heard the term "trench" (related to woodworking) was in a YouTube video by Peter Parfit. Seems like another of those things that are said differently in the UK, but have enough of the same meaning that you understand.
There are others too. In the US the thing we would call a router bit, would be a cutter in the UK.
We also refer to the machine that turns drill bits as a drill and, as far as I know, they drop the bit part in the UK. A drill is the cutting part. (the machine is a drill motor?)
Here we would use trench to mean a narrow groove dug in the ground, to bury a cable or pipe, usually filled back in. Left open would be a ditch.
None of this is set in stone, just general common usage, possibly even regional
In this context, it seems like they are referring to a fairly small diameter straight bit?