Er... While the original idea for embassy plots was indeed to be town fronts in spawn, that appeared to be directly intertwined with the fact that the GP plugin had no town correlations. It would have been a requirement to have the embassy plot before making a town. Since that is no longer the case (insert Towny), I made the assumption that 'embassy' was just the name of plots on the lowest tier of central spawn for players to purchase and additional 'non-central' 'noble' plots would come out later with the port cities.
If that is still the intention, yet reversed (make Towny town -> buy plot at spawn to promote town/vault storage), then logically all trade area/central spawn plots should be through PE purchase only, as to be verified through an admin that the purchaser is indeed the mayor of a town.
Regardless of any of that, all the information I found on the matter of plots in spawns (aside from The Slums) have held the same rules. The build must be well designed. I can't imagine why the rules would seem unclear to anyone. In italics right under the section header it says 'spawn plots', which implies any plots at spawns and I would again make an assumption that that was done to futureproof the wording. (I.E. the day someone goes and names a tier 'embassy', which oddly enough, I found references to 'embassy' plots in a post for...Haven, I think? Anyway...)
What is unclear, however, is why the term 'embassy' tier/plot was kept (or continues to be used) when Towny was decided upon while Towny has a specific embassy plot permission that would be conflicting and confusing. I have not yet made a town but the wiki is currently set up with terminology that supports the Towny plugin '/plot set embassy' command. That command, as far as I could tell, is simply a way for towns to rent out plots of area with different set taxes/to non-town members. Not at all aligning with any GP embassy plot wording.
Sooo... The issue there isn't really rule clarity, it's clarity regarding the name of the tier of plots located on the lowest level and how that conflicts with previously stated, and now scrapped, plans regarding things called 'embassies' for the GP plugin.
Of course, a simple addition of the word 'Embassy' to the section header in the rules would suffice for those that can't make the leap regarding the cobble box featured in the OP, but there appears to be a few people who still think those are town-only plots, embassies for towns, which they clearly are not, as there are more towns than plots currently owned.
Cobble boxes wouldn't be acceptable even if the tier was called The Poopy McStinker Level of Boring, because they're still spawn plots and the rules for the past 2 years (That's as far back as I could find) haven't been different unless otherwise explicitly stated.