He may have had in mind things like Howard Shore’s Lord of the Rings Symphony (that GIGObuster mentioned), which was constructed from the film scores.
“What counts as a symphony?” is a reasonable question, but the simplest answer for our purposes may be “Anything that’s called a ‘symphony’,” at least as long as there’s no good reason to rule it out.
Another possible contender: Malcolm Arnold’s 9th (of 1986), which this article claims is “a neglected 20th century masterpiece.”
Recent superb symphonies, since (let’s say) 1989. This is not an exhaustive list.
Per Nørgård: Symphony No. 8, 2011(I also like much his Sixth (1999) and Seventh (2006))
John Adams: Dr. Atomic Symphony, 2007 (yes, the music is from his opera Dr. Atomic, but it works great as a symphony and he’s not the first composer to do this.)
John Adams: Naive and Sentimental Music, 1999 (a symphony in all but name!)
Elliott Carter: Sinfonia, 1996
Witold Lutosławski: Symphony No. 4, 1992
Shulamit Ran: Symphony, 1990
Poul Ruders: Symphony No. 1, 1989 (he’s written three more, but I only know No. 2 in addition to No. 1, and No. 2 struck me as not as good.)
My own Symphony No. 1 is forthcoming…(for real, I’m a composer professionally.) We’ll see how it turns out. I expect it to be bitchin’. Fall 2016 is the likely time for completion/premiere.
As for truly great piano concertos post-Rachmaninoff, there are almost too many to name: Bartók (all three!), Ravel (both the G major and the one for the Left Hand), Prokofiev (No. 3 for sure but all 5), Schoenberg Op. 42, Shostakovich No.1 and No. 2, Barber, Copland, Gershwin, Lutosławski, Ligeti …
(Disclosure: I’m one who greatly prefers Rachmaninoff’s choral music and mostly dislikes his concerti and piano solo oeuvre.)