Honestly, there's evidence for both theories. Enterprise twice said that the future was changed, first when a colony was destroyed in "Shockwave" and again when the Xindi attacked - yet season four was full of TOS prequel episodes. Enterprise contradicted TOS a few times (cloaking devices etc) and the NX01 wasn't on the TMP or TNG Enterprise legacy walls.
I personally stick it all in one timeline up until the Kelvin attack at the start of STXI, but I don't freak out at continuity errors (although I get a kick out of pointing them out!)