Sunny was aware of the existence of the old Equestria map thanks to her dad, but she was made to believe that every Equestria city, besides Maretime Bay, Bridlewood, and Zephyr Heights, was abandoned or destroyed. Whelp, some pony told her that some of the cities still exist and they're as functional as ever. This makes Sunny get excited, to the point that she can't maintain herself. Charlie seems to be present for some reason. Judging by her name tag, she was at work and came to downtown Maretime Bay by tram for a lunch break, perhaps to order a smoothie from Sunny. But that turned rather interesting fast.
Sunny will be happily telling her friends later, although, Zipp may not be surprised by what Sunny says, but the others will be.
Golden Age of Equestria is the term to what the canon G5 refers as the Ancient Equestria, but in my Universe, the proper term is called the "Golden Age Of Equestria" because that was the era of peace and harmony, basically during Princess Celestia's ruling to the era of the Mane 6, and lastly, Princess Twilight's Sparkle's ruling/reign. This was the only true time Equestria was really in peace and despite Twilight and her friend's efforts to enforce harmony and friendship across Equestria and other kingdoms, the time of peace ends when Twilight's friends pass away and that is when the corruption slowly occurs until the three pony races loose their magical abilities. Sunny was only taught snips and pieces of this.
Disclaimer: Don't take what I claim as fact, this is apart of my alternate universe. What I may implement in my art and stuff is most likely not canon at all. This is not a headcanon.