I understand your point, but that’s what this law would theoritcally accomplish. It would limit representatives/senators from being able to trade stocks (a higher standard than an average citizen). While they can buy/sell stocks before the law, that’s just the current state of affairs.
Regarding the timing of trades, etc. The value of their insider knowledge is really only beneficial over long periods of time. Yes they could buy/sell stock in the interim, but preventing it long term would be an actual win.