It is quite possible that, as snailplane suggests, the writer simply made a mistake which was not caught in editing.
I believe, however, that Man_from_India has caught exactly what is happening here. There are two parts to the analysis he suggests:
... the writer is considering “filming and editing” to be a single task.
This is a very common use. In my own shop we say ‘shooting and editing’, which amounts to the same thing, and we frequently have occasion to employ this phrase in the singular to designate a particular phase of video production. For corrobaration, consider these examples from Google:
Our video filming and editing service will be carried out by our students under the supervision of academic staff at the university. —University of Hertfordshire Note that ‘filming and editing’ modifies a singular noun, service.
Filming and editing is the meat and veg of video, whilst post-production is the garnish and presentation that makes it really shine.
It's not the years that is getting finished. It's filming and editing ...
I suggest that years of is being used here analogously to lots of:
Lots of work has been finished already.
Lots here is what CGEL calls a ‘number-transparent quantificational noun’. I have discussed such constructions in more detail here; briefly, these are constructions in which the number of the entire noun phrase is determined not by the number of its formal head (in this case years) but by the number of its ‘oblique’ (filming and editing, which as we have already seen may be understood as a singular phrase).
Again, this is a common use.
All at once 7 years of work was gone.
In that moment, 33 years of waiting was over for the Dahmers.
The last three years of study has been very rewarding.
Other measures work this way, too:
Six pints of water is not an excessive amount.
About 294 million board feet of lumber was manufactured in Montana during the first half of 2014.
Nearly 10,000 man-hours of work has been executed for this client.
It's filming and editing that spans over a couple of years.