That is incorrect. The rules for Modifying Characteristics are very clear in the Core Rules. The source of the modifier is irrelevant, all modifiers are applied at the same time in the listed manner:
Warhammer 40,000 Core Rules: MODIFYING CHARACTERISTICS wrote:Many rules modify the characteristics of models and weapons. All modifiers to a characteristic are cumulative; you must apply division modifiers before applying multiplication modifiers, and before applying addition and then subtraction modifiers. Round any fractions up after applying all modifiers. If a rule instructs you to replace one characteristic with a specified value, change the relevant characteristic to the new value before applying any modifiers that apply from other rules (if any) to the new value. Regardless of the source, the Strength, Toughness, Attacks and Leadership characteristics of a model can never be modified below 1....
Example: A Space Marine Sergeant (Strength characteristic 4) is making an attack with a power fist (Strength characteristic x2) while under the effects of a psychic power that increases his Strength characteristic by 1. The two modifiers (x2 and +1) are cumulative and applied concurrently. The attack is therefore resolved at Strength 9 ([4x2]+1=9).