no, they're not.

there were nomi monsters before AST (exodia necross, berserk dragon, dark paladin, mirage knight, just to name a few). and there have been plenty of non-nomi's since AST.

nomis are determined solely by their text. special summon only AND nomi monsters start with the sentance "This card cannot be normal summoned or set." which simply means exactly what it says. the next sentance determines if its special summon only or if its nomi.

special summon only monsters say "This card can only be special summoned by...."

nomi monsters say "This card cannot be special summoned except by...."

No...don't forget that Fusions and Rituals (that aren't Nomi) also fall under the Special Summon-only status as well...

Master of Oz
Ojama King
Sanwitch
Dark Blade the Dragon Knight
Mokey Mokey King
Gatling Dragon
King Dragun
Reshef the Dark Being
Elemental Mistress Doriado
UFOroid Fighter
Cyber Twin Dragon
Cyber End Dragon
VW-Tiger Catapult
VWXYZ-Dragon Catapult Cannon
Cyber Blader
Ruin, Queen of Oblivion
Demise, King of Armageddon
D.3.S. Frog
E. Hero Aqua Neos
E. Hero Flare Neos
E. Hero Black Neos
Rescuecueroid/"Ambulance Rescueroid"
Super Vehicroid - Jumbo Drill
Cyber Ogre 2 (CDIP)
E. Hero Air Neos (STON)
E. Hero Groun Neos (STON)
E. Hero Glow Neos (STON)

Well okay, besides rituals and fusions.

Still, they count as "Special Summon-only" monsters and fall under the same, general, category as non-Fusion and Ritual monsters.