This doesnt change the Halfling or WE ability at all (both abilities only change what you can hide behind; neither ability changes the restriction of not being able to hide while being observed).
So you can how hide when a creature 'cant see you clearly' instead of only being able to hide when you cant be seen at all.
I suspect that's all this errata is doing... clearing up one possible hiccup where people might be wondering about their judgement call-- especially when in regards to the Wood Elf and the Halfling's abilities.
When a Wood Elf is in natural terrain and Lightly Obscured, they were allowed to make DEX (Stealth) checks to hide. Now by definition lightly obscuring terrain doesn't completely block line of sight. If it did, it would be considered Heavily Obscuring terrain. And thus, a player could make the argument that a wood elf in lightly obscuring terrain could possibly been seen in some form or fashion.
Thus... when you run this state up against the original rule for going into Hiding-- which is that you aren't allowed to be seen AT ALL (because it says in black-and-white "can't be seen" as opposed to the current errata of "can't be seen clearly")... perhaps some players have been questioning them how Wood Elves are allowed to make the attempt? Being in Lightly Obscuring terrain allows them to try and hide, but yet they haven't fulfilled Rule #1 in hiding, which is they cannot be seen at all.
Now sure... a so-called "common sense" ruling by the DM would have him make the jump in logic that the whole point of the 'Mark of the Wild' rule is to change the "can't be seen" definition of Hiding to "can't be seen clearly because Lightly Obscuring terrain doesn't entirely block line of sight and thus the Wood Elf theoretically could be noticed". But if not enough players were making that jump from A to B... perhaps that's why they added the "seen clearly" amendment via the errata.
This is the stumbling block that comes with writing rules in Natural Language-- some player's use of the language isn't always that natural. And thus, the few times when it seems to them that enough people aren't putting 2 + 2 together to make 4, they have to change their natural language a bit to help them solve the equation.