The law in both the USA and the EU requires you to list all potential sources of allergenic proteins in ingredients, additives and processing aids (when those allergens could potentially be left in the product in ANY amount). The law does not exempt certain levels of allergens. For soy, they should be listed on the label regardless of the expected amount present.
Only sulphites and gluten have specific limits in EU law.
The chocolate company says the levels present are insufficient to cause a reaction in the majority of soy allergic consumers, but they should be labelling it to protect ALL soy allergic consumers, INCLUDING the ones that would react to the small amounts present. While they deem the risk of occurrence minor, the potential harm (aka death) is so high that the overall risk is very much significant.
References:
Food Allergies | FDA
EUR-Lex - 32011R1169 - NL - EUR-Lex (europa.eu) (Article 21 and Annex II)