[maxerickson]

The thing driving the regulation is that there is an (small or trace but) unknown amount of sesame.

If it was obvious it was none there wouldn't be an issue.

[s1artibartfast]

So the solution to comply is to intentionally add Sesame to the manufacturing process has the most effective way to meet the regulation. This means that products that previously may have had occasionally low PPM levels of sesame now have a much greater amount intentionally added in larger but known quantities

[kragen]

in some cases there were trace amounts, and in others there weren't, but the absence of sesame couldn't be guaranteed

we don't know specifically which cases there was no sesame before but we do know there were a lot of them