How is the rook on f2 going to be saved after the played 19...c5 move?
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Without the rook check, the bishop on f4 could still move to g3.
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Allowing the rook capture also allows the White rook to be captured (White cannot get any compensation from that loss afterwards), effectively allowing a trade of Black rooks into a simplification for Black's win.
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In other words, whether you saw the 18...Rc1+ move up to the capture of the f2 rook or not (i.e. 18...Rc1+ was played as an "error" due to you not noticing the f4 bishop), it was winning. I suspect it's labelled brilliant because you played the move 18...Rc1+ more quickly than the computer analysed it.
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EDIT: Missed that you mentioned that "you forgot about the bishop". That said, that was an entirely winning move that you have played, and more than just winning the rook, a mate would soon arrive! Kudos! I would not have seen that winning move 18...Rc1+ in an actual game within a short time frame. I had to confirm with Stockfish that the best line of defence for White (giving up the c1 bishop on the e3 square) still allows Black to land a checkmate soon after.
I might just need to understand what Brilliant actually means. I'm fairly a nube, so please have patience with me.. Why is move 19...c1 black, marked as Brilliant? I forgot about the Bishop, and moved right into loosing that Rook with only a momentary check on a fairly guarded king... seems more like a Blunder to me.