Yup.
The Millies were not just Trots who thought it was prudent and useful to be in the Labour Party. For them, being in the Labour Party and working through the Labour Party was an article of faith, an absolutely key tenet of Millie Trottery. They fetishised it. Any would-be socialist who was not in, or organised any campaign or protest without going through, the august organs of the Labour Party and its affiliated unions was denounced as ultra-left, petty-bourgeois blah, blah, blah.
After their expulsions/resignations, the Millies performed an 'about turn' as smartly as any bunch of well-trained parade-ground squaddies. They still care deeply about the issue but now the key point is that the Labour Party is no longer a workers' party. The Millies (or Socialist 'Party', as they call themselves) retain all their fervor and dogmatism, but now use it to denounce anyone who (however moderately) believes that it's worthwhile working in the Labour Party.
"Trotlets, abouuuuuuuuut..... wait for it, wait for it... abouuuuuut turn!"