You can't use a scroll with a bonus action because the rules state "Casting the spell by reading the scroll requires the spell’s normal casting time."
Yes, that is the general rule. But remember, from the PHB:
"Exceptions Supersede General Rules
General rules govern each part of the game. For example, the combat rules tell you that melee attacks use Strength and ranged attacks use Dexterity. That’s a general rule, and a general rule is in effect as long as something in the game doesn’t explicitly say otherwise.
The game also includes elements—class features, feats, weapon properties, spells, magic items, monster abilities, and the like—that sometimes contradict a general rule. When an exception and a general rule disagree, the exception wins. For example, if a feature says you can make melee attacks using your Charisma, you can do so, even though that statement disagrees with the general rule."
Even if you could, it's still just delaying the attack to another turn by taking the ready action. Again, I don't see why it helps. True strike doesn't grant advantage so it doesn't guarantee sneak attack. It's less useful than using a bonus action to hide and ready a ranged attack.
I'm not sure if this is an issue with the people explaining the move writing poorly, or the concept being hard to understand. So,
in details:
Step 0: be a thief/level 1 dip caster and have one or more scroll
Step 1: Be in a position where a sneak attack is almost garanteed - your foe is in base contact with an ally, for example.
Step 2: use the fast hand ability to cast true strike as a bonus action.
2024 true strike "you make one attack with the weapon used in the spell’s casting" . Hopefully this attack lands and you deliver sneak attack damage.
Step 3: Ready an action - to attack the same target - and set the trigger to be something you know will happen almost immediately after your turn is done (if you know the initiative order, this should not be hard).
Step 4: the action is triggered, take the attack, and if you hit, once again deliver sneak attack damage
tada, 2 sneak attacks in a round!
So I still don't see the point. True strike doesn't guarantee sneak attack, you can't cast it as a bonus action off a scroll, ready is still an action.
True strike does not guarantee the sneak attack no, you have to engineer the situation. But it's not very hard to do so. And since the exception (the thief's ability) trumps the general (casting a spell on a scroll takes its normal length), you
can cast a spell off a scroll.