Well let me take a stab at it. The line in question is said to be a probe of an Fe XI line, that is iron atoms with 10 electrons removed.
You do not get such ions in the solar photosphere, it is far too cool; the radiation from the photosphere is probably a pseudo-continuum at that wavelength.
However, much hotter material in the chromosphere and corona may contain Fe XI ions. Plasma in these structures could absorb light from the underlying photosphere, if it were dense enough, or more likely, if you point your camera above the limb of the photosphere, you could see optically thin coronal structures emitting light at this wavelength.
Why is this important? Normally you would need to use EUV or X-ray emission to probe the coronal plasma, but the imaging quality is not so good. You can do much better at optical wavelengths, but there are precious few diagnostic lines that can be used.
Edit: Indeed this presentation on DKIST coronal diagnostics appears to confirm this hunch and also mentions the additional possibilities that polarimetry of optical/IR spectral lines offer in investigating coronal magnetic structures. The Fe XI line in question is sensitive to Zeeman splitting, offering the opportunity to prove the strength and direction of magnetic fields where the line is formed. The Zeeman effect is proportional to the square of the wavelength, so the more usual X-ray and EUV diagnostics just can't do that.
On p.2 of that presentation it clearly states that these kinds of measurements are limited to coronagraphic images taken off the limb of the Sun. Pointing at the photosphere will not yield useful information for these diagnostics, because the weak light from the chromosphere and corona is swamped by the normal photospheric emission.
Sextus Empiricus has pointed to a press release, which although unclear in its exact meaning, implies that the dark features around the granulation may offer sufficient contrast to see the much thinner and hotter chromospheric gas directly above, producing bright points in the dark lanes. Observing these through different polarising filters could then reveal details about the structure and strength of the magnetic field.
However, on further research, these photospheric bright points are nothing of the sort. They are concentrated magnetic flux tubes that allow a deeper (and therefore hotter and brighter) view of the Sun. The typical temperatures of the deeper material is still only around $10^4$ K (e.g. Shelyag et al. 2010) and nowhere near enough to excite Fe XI.
The picture below, which comes from this site, referred to by Sextus Empiricus, shows the situation. A bundle of magnetic flux "hollows out" a passage further into the solar interior and the light we see comes from deeper, hotter, brighter regions. Nothing to do with coronal emission.
I arrive at the conclusion that this image (which was taken for science verification purposes) was just using the Fe XI filter as a narrowband filter. Everything we are seeing in the image is essentially continuum from the photosphere at temperatures between 4000K and $\sim 10^4$K. The contrast therefore just arises from the differing monochromatic intensity of material at different temperatures.