"Interlude Party" is a recap/clip episode designed to catch up viewers who may have missed the preceding episodes.
The episode is structured as a dream Hohenheim has in his travels, where he revisits past timeline events through interaction with a very old friend of his (Pinako). He knew her when she was much younger, so it is not unreasonable that, in a dream, he would see her as her younger self.
Nothing about the timeline is messed up; dreams, in both fiction and reality, don't necessarily maintain temporal consistency, and the same is true here. Hohenheim knew Pinako as a young woman, and he knows what his sons have done. Therefore, in a dream, a young Pinako can indeed talk about events that occur when she is an old woman, with no damage whatsoever to the timeline of the waking world's timeline.
The dream nature of the events are made clear several times throughout the episode (characters abruptly changing age, or knowing things that would not yet have come to pass at their depicted ages); furthermore, it is emphasized at the end, when Hohenheim wakes up.