well...The book-matched pattern would be indicative of veneer for the center and i would stick with that as an indicator of such, better to be safe than sorry. If the bottom is clear, you can also just look and see if the same pattern is like the top, if not then it's definitely veneer, normally only the face is done in such manner.
Since Alcohol can attack a alcohol soluble dye in the wood i won't completely rule that out as the cause, but i will say that no alcohol damage i have seen would leave the mark as such. Normally if alcohol were to attack the dye it would leave a darker edge on the utmost boundaries of the stain, and i see no evidence of that, the edges look as light as the rest.
To me, and this is just an educated guess, something such as a cloth material was laid upon it for an extended period of time, maybe a cloth belt from a bathrobe etc., That i can't determine, could have been just a long strip of cloth. Since there is also no clue as to why, or what is was being used for if anything, especially so carelessly, it may have been the work of a child.
whatever it was had to have been pretty wet so as to affected the damage so quickly without drying out and or the finish itself in poor condition to allow the liquid to pass through to the wood. Maybe in this instance, the stain is a type of bleach itself. Without being there to test, that could not easily be determined.
The only thing i can offer is chemical assistance as to trying to give a uniform look to the wood in order of lightning the entire surface, but again with no guarantee of the final results. If by chance it is a bleach that has caused the mark, further bleaching will most likely continue to lighten the area more so than the rest, and the difference in color between them remain. Both being lighter in color, but with the same contrast of light and darkness.
In this case also normal household bleach or even a stronger sodium hypochlorite bleach such as used in swimming pools [5 vs 25%] would not do it, the available nascent oxygen not being enough to produce a like affect as seen on the damage as it exist in the pics. Though it may help blend it in better than as it exist now.
2 part bleaches [AB - 1/2 ] will definitely work as to making the top look even lighter than the damaged area, but also will completely hide the natural beauty of the walnut in doing so.
The other problem is not having a sample to at least experiment on to determine what would be best in this case. running test on the surface itself may do more harm than good, sticky situation here.
With this all in mind, i would suggest that you keep sanding the area as well as the rest of the top to keep it staying uniform, with all due caution to not sand through the veneer and create yet another problem. With this, it would be good to know your finishing abilities as they stand and if your capable or know how to touch up and blend if variances in color?
If you can, that would be the best suggestion. Sand as much as you can, re-apply the desired stain/dye, seal / sand lightly with 320 or 400, re-apply clear and with an artist brush and stain color blend in as well as possible, let dry and re-seal once again. note the look and either continue the color repair if necessary or apply your final coat.
Note: always keep in mind that when applying the same stain you use on the bare wood, that when sealed will not give the same color and will need small adjustments.
Other ways can work , i will leave that to others to reply back to you ok?