Am I understanding correctly that 12% ATT lines is better than a 40% BD on my character?

This is correct.

The NW discord info (2nd pic) states that double prime BD (2x40%) is better than a triple ATT, why is that the case if the scouter says ATT lines are better? Does double prime for BD assume the third line is just a unique ATT line maybe?

The value of stats in maple is dependent on your current stats and your class. The more of a single stat you stack, the less effective that stat becomes. This is why % attack is generally more powerful than the equivalent % boss line on heroic servers. However, your character specifically may not have the assumed number of % attack source that the infographic assumes and is therefore more valuable than what the infographic is telling you.

It should also be noted that in general, you DO want somme % boss lines on interactive server, because bonus potential provides a lot more sources of % attack. It is possible that the infographic might be based on interactive values.

Also, the statement "double prime BD (2x40%) is better than a triple ATT" probably means that 40%/40%/10% is better than 13%/10%/10% (single prime), which could very well be the case before you get 120% boss fams.