The reasoning behind this is that it appears that all Pokémon that were used in the demo had fixed IVs, EVs and natures. And it just so happens that a level 100 Mewtwo had 353 HP there, which strongly suggests max IVs but no EVs. Should that be transposed to other species, we can find out that since a level 30 Sylveon had 106 HP, its base HP should be 95, which just so happens to match the typical 60-65-65-95-110-130 scheme Eeveelutions always have.
Working on finding more data to work on, to figure out the base HP of other new species. (I still can't believe NO ONE went to take a look at the other stats in the demo. Aren't they the slightest bit curious?)
Edit: Should this pattern carry over, then this means that Chespin has base 57 HP, Fennekin has base 40 or 41 HP, and Froakie has base 42 HP. For reference purposes, compared to the other starters, this means Chespin is a distant second to Tepig, while Fennekin and Froakie both have very low HP. As for Helioptile, it's either 45 or 46.
Edit 2: Apparently the buttons to see the stats were greyed out. Shame, I was really interested in seeing what those Mega forms had in them.