Both sources are Blizzard's own, so everything here is confirmed. The video quotes come from its automatic subtitles; the article quotes from the article itself.
Why rulesets instead of realms
"Not where you play, but how you play," the video says. The goal "isn't to build up one giant server. It's to let like-minded players link up in game more easily." There are four at launch: Normal, PvP, Roleplay and Roleplay-PvP. The video itself does not show the last one, but its description says Blizzard is "extremely excited to add RP-PvP to that list". Roleplay has its own copies of the outdoor world; players there can still flag for PvP.
Choices that stick
PvE and PvP cannot be swapped later: "The restriction is what makes the choice mean something." On PvP the faction is locked as well: "Roll an orc under PvP and every PvP character you create from then on will be Horde. There is no second PvP space." The article says the same - players on PvP rulesets are restricted to one faction. If one side grows too large, Forever "introduces temporary restrictions on character creation".
Economies that never mix
Auction house, guilds and economy belong to a ruleset. "Any PvE player can trade with any other PvE player in their region... but those economies never mix." Groups, guilds, dungeons and raids only work within the same ruleset and the same faction; cross-faction parties, raids, guilds and battleground teams do not exist.
Hardcore
Hardcore is not available at launch and will stay sealed off. A character that dies there "can be sent to any of the three non-hardcore rule sets" - with one limit: not into PvP if you already have characters of the other faction there.
What the guide adds
The article on what is new also describes the two experience presets, which change presentation only and can be undone under Options, and repeats that Legacy has three tracks: Professions, Adventure and Resourcefulness. One subscription covers modern WoW and Forever.
What this changes here
The rulesets page gains the faction lock on PvP and the limit for fallen Hardcore characters. Our item on the launch overview video had the third Legacy track as "progression", taken from the subtitles; it now says Resourcefulness.