Stalemate and checkmate both involve the player to move having no legal moves. The difference is that, with stalemate, this player is not in check, but with checkmate, this player is in check.
You are correct that the Black king cannot move and, since Black has no other pieces, Black has no legal moves. But Black is not in check. Therefore, this is stalemate and not checkmate.
As Glorfindel has pointed out, if it was White’s turn instead of Black’s turn, it would be possible to force checkmate in two moves. That answer also pointed out that White could force checkmate with just one rook, as described in Wikipedia. This procedure is also described in Wikibooks, along with other useful endgames.
Finally, note that excessive promotion like this (you must have promoted at least two pawns to rooks) is usually discouraged, precisely because it often leads to stalemates like the one you accidentally created.