I think she wore something completely appropriate. I myself have worn an open cardigan/jacket with a dress as it tends to hide a first/second trimester bump or even a spare tyre or two underneath very well. It makes it much less obvious. You can hold the coat or jacket strategically to prevent a side view or even a full frontal view.
Any dress of any colour on its own would have clearly shown the bump that was visible a few days later, especially if she had opted for a slim fitting dress which is the style she normally favoured. If she had departed from that style and chosen a looser style, tongues would have been a wagging too, and on top of that she would have risked a wind-related skirt incident on such a blustery day.
Navy dress with coat closed would also have shown the bump.
There was speculation from the day they were married that they would be trying for a baby asap, given her age, Harry's professed fondness for children, etc. No matter what she had worn I think people would have speculated. She wore a vertically ruffled dress at one point about a month before the wedding iirc, sparking much excitement.
Then after the announcement there was talk of twins, and now there is even speculation about when their next child will be born.