That sounds reasonable - much better to announce a "nine month" delay now (compared to opening in December 2018) and then try to do it sooner - the worst thing in such projects is to announce a two/three month today, then a further two/three month delay in a few months time, then another short delay... losing confidence with everyone involved - much better to give everyone (including the people planning the parallel bus routes, not just the rail industry) breathing space - and a "season" rather than a "month" deadline is nice and nebulous.
Might not be bad to have a "soft launch" independent of the six monthly railway timetable changes too.
I do know that coming up with a budget and delivery schedule for such a mega-project seems to be a hostage to fortune - there's no simple benchmark for something of this size under central London (the Jubilee Line extension is the closest I guess, but that was mainly an extension outside of the centre, rather than mainly directly through the centre).
I don't know if there are penalty clauses - maybe there are - maybe not - but that's something to discuss internally - and often you find that firms "lawyer up" and it becomes incredibly expensive to reclaim money - you might sue them for a million, be awarded half that and find you've encountered hundreds of thousands of pounds in legal costs - so there's an element of it not being worth doing for the sake of maintaining relationships - especially as putting in big penalty clauses invites contractors to bump up their fees in the anticipation of having to pay out penalty clauses - it's not free money.
But, in our Scapegoat Society, I wouldn't be surprised if someone demands a pound of flesh from one of the contractors. It won't help, but it'll make us all feel better if we can get some "Fat Cat" to lose their job because the mob of angry people must be placated.
I'm going to report this post for a breach of Forum Rules (subsection ninety three - "thou shalt not criticise the perfect world of German/Swiss railways")!