Last year a huge game company released something built on my tiny open-source game engine (uncredited), and I only found out about it later from a kind internet stranger. All things considered, better to know in advance so you can at least have your own response ready, so you can comment in the relevant HN/reddit threads, etc.
That said, the "keep it secret" part of the mail here does sound weird, but given the other history there may have been an NDA in place.