I think that they are playing a very dangerous game. Letting Farage preach from all platforms and then mimicking his soundbites might push Reform and the Tories in a "who is more right wing" competition (e.g. "shoot'em on the beaches"). At that point the moderates will flock to Labour.
Of course the left side of labour are ignored because their votes are taken for granted and, in the worst case scenario, greens and libdems would form a coalition with labour to stop the far right and, in any case, they are not seen as a threat, coming the time of the next elections.