A patch of land is unusable while mined; useful when cleared, but if it's 50% cleared (or 80% cleared) then it is murderous, because then it gets used, resulting in dead and crippled locals.
A field that you must use to avoid starving has 100 mines in it, you can remove [on average] 55 of those mines using a locally made and affordable device. The field will be used anyway. Would you deploy the device?
You're leading a demining team. A village have asked you to clear a field that they rely on for crops (other areas are too rocky or haven't enough good soil). You can send out some locally produced cheap demining devices first that will remove about 50% of the mines reducing the risk to your team. Would you deploy the device?
"is not useful" you say?
Your position appears to be a counterpoint to a statement that these devices will clear all mines effectively from all areas in all conditions. I've not seen anyone make such a claim. Your position is only tenable when a mined area is not needed by the locals.
The devices, and the beautifully made film, have another purpose which is far from useless too - attracting attention and thence funding for demining projects.
The argument for partial demining leading to increased harm requires that land that is mined remain completely unused otherwise or be completely demined. As soon as there is any use then reducing the number of mines statistically [and yes maybe simplistically] will be better - which would you rather, walk across a field of 200 mines or 400 mines?
Professionally demined areas are only cleared to a level of about 99.6% according to the thread so the argument for increased risk of harm still works in that case - more risk from mines when walking in a 'completely' demined area than in an area in which mines have never been placed.