Your interpretation is not without merit, but lets look at the consequences of it:
1. Gaining the Slow trait after making a move action would not have any impact on the current Action Phase at all then. You would still be able to make a quick action, including a second move, because none of it is written with regards to the initial move being in the past tense.
2. This logic would also prevent Charge +X from working if it is applied mid Action Phase. There may be other traits that also become ineffectual unless revealed much earlier than currently possible, but I haven't scoured the rulebook for them.
From discussions with Arcane Wonders and reading the Rules as well as the Rules and Codex Supplement, I assure you that this is not how they are meant to be interpreted. Both of these effects care about whether or not the creature takes a move action this action phase, not whether they take one after the trait is applied, when determining whether or not they have an effect. Perhaps they could be better written to convey that, but that is what is meant.
