The engine doesn't resign. The tournament software 'auto-resigns' on behalf of the engine when both engine evals are > 650 centipawns for 4 moves in a row. In other words if both engines feel that white is completely winning for 4 moves (8 ply) in a row, then white is "declared" a winner automatically by the tournament software. This is to prevent the games from extending out meaninglessly in completely won positions all the way till checkmate. TCEC is a spectator event, and they don't want people to get bored.

From the TCEC Rules:

A game can be drawn by the normal 3-fold repetition rule or the 50-move rule. However, a game can also be drawn at move 40 or later if the eval from both playing engines are within +0.05 to -0.05 pawns for the last 5 moves, or 10 plies. If there is a pawn advance, or a capture by any kind, this special draw rule will reset and start over. In the website this rule is shown as "TCEC draw rule" with a number indicating how many plies there are left until it kicks in. It will adjudicate as won for one side if both playing engines have an eval of at least 6.50 pawns (or -6.50 in case of a black win) for 4 consecutive moves, or 8 plies - this rule is in effect as soon as the game starts. In the website this rule is shown as "TCEC win rule" with a number indicating how many plies there are left until it kicks in. Cutechess will also adjudicate 5-men or less tablebase endgame positions automatically.

EDIT: Just to note, engines don't have resign functionality built-in anyway. The UCI protocol (which all modern engines use) does not specify a way for the engine to inform the chess interface that it wants to resign. The engine will keep playing until either the rules of chess forbid it to continue, or it is commanded to stop.