After skimming Gates, I think where the issue hangs up is there have been discussions in prior cases of what the principal residence in section 121 means. And in those cases the excludable sale must include a residence in order to be considered a sale of one's principal residence, specifying that a principal residence must include a house and/or a home at the time of sale. If the house is demolished or removed, you're no longer selling a residence and can't exclude the income.
In Bogley vs. Commissioner cited in Gates, it was ruled that a person can sell one's residence with part of the land, and subdivide out and sell another part of the land and include the gain from that subdivided land in the exclusion, as long as the subdivided land had also been used as the residence. The court also considered that the subdivided-land sale occurred within a year of the home sale. Again here, the deciding factor seems to have been that the home, the essence of the residence, was never demolished/removed, retaining the nature of the sale proceeds as being from the sale of a principal residence.
Also in Hale v. Commissioner cited in Gates: “The sale of a taxpayer's residence requires the sale of a structure which is used as a principal place of abode, and we have held that the sale of land without the structure does not constitute a sale of a residence within the meaning of section 1034.” This isn't directly related to 121, but bears on it.
From the Gates conclusion: "If a taxpayer sold some part of the underlying land but not the dwelling that the taxpayer used as a principal residence, the taxpayer could not defer the recognition of gain on the sale because the taxpayer did not sell his or her principal residence. . .If petitioners had sold their old home instead of demolishing it, they would have qualified for the section 121 exclusion. That is not what they did."
It was a majority decision, not unanimous though, and 4 judges dissented. The dissent agreed with the majority that the sale must include a permanent dwelling and that a principal-residence sale must include a home and improvements, but argued that the dwelling could be demolished and rebuilt as the petitioners had done, where even though the original house was gone, this would be defined as renovation of the original principal residence, not as a different residence, and not interrupt the 5-year period.
I guess the exclusion just isn't favorable to manufactured or mobile homes. It's understandable, since one can gain property tax advantages by living in one of those as a non-permanent structure, yet can't 'double dip' by having tax-free land gains just by parking a trailer on a plot of land for 2 years at a time.
If the state was offering to buy the land with the structures on it and perform the demolitions itself, I think it would be excludable. But that's not what the offer is, it's for a vacant land purchase only.