I suspect it is more a function of TSB never having had do do any complex IT work for themselves since it was all done by Lloyds at arms length and even then basically no projects had been done by TSB since it was carved out of Lloyds.
So their management had no idea how to manage technology in what is essentially an IT company. But they didn't recognise their own lack of knowledge in what is probably the most complex It project ever done at a UK bank. Sabadell should have called it out and TSB should have recognised their own lack of skill. Both to blame.