I don't think this is a particularly lucky usage of Royal Seal.
Here's my thoughts on Royal Seal in general:
I think of it as a treasure version of Merchant Ship early, and a Silver late. Both get you $2 on this turn and a bonus next turn. Merchant Ship always gives $2, and Royal Seal in a sense gives you (utility of newest card)-(utility of average card in draw pile), which will often be pretty close to $2, and be better when you buy platinum or multiple cards that combo together. The reason Royal Seal typically seems weak is that there are often not enough plays before you start greening and it turns into a simple Silver. It suffers the same problems as the other "weak" cards from Prosperity -- it basically needs Colony games.