Basically it clarifies.
If you have/had windows 7/8/8.1 license and want to upgrade to Win 10 it is free for you.
This is the same what they have been saying since the beginning.
Now if you happened to upgrade your version of win 7/8 to the windows 10 preview, you have the full version free also.
IF you have installed preview version without any previous license, you will have to pay.
Fair enough, since you did not buy windows 7/8 previously and they sure will not hand out Win 10 to you free of charge just because you happen to test a preview build.