Whatever Picard or Locutus knew, the fact of the matter is that he appears confused by the antimatter fireworks.
Probably no such thing was included in the original plan that Picard was aware of. Quite possibly Picard had never even considered the possibility that his ship would be capable of deploying these fireworks - it may be a Shelby-devised procedure. Whatever the specifics, the deployment of the antimatter spread makes Locutus look confused, whereas the saucer separation as such evoked no such body language.
The Collective, not Picard. Picard wasn't in control of his body at that point, so nothing that he was feeling could have produced an expression of emotion.