Siemens are well able to secure orders from other countries who would like delivery of proven, well built reliable modern rolling stock and their order book is not exactly empty over the next few years, so it's no water off their backs really, they're not going to spend huge amounts of money fighting that kind of battle even if they have just cause to do so because the UK is not going to be importance to them that much going forward;.
At the end of the day they know that once Brexit happens it will not be a positive change for them since it will allow orders to be put through by politicians who would like to make political gain out of them and instead of wasting their efforts on a UK market where they're likely to have a much smaller part in a post brexit country,. they'll probably just put the effort into countries who view quality as more important than nationality.
Of course, if the UK voted remain, it might be a very different story and I'd be curious when the decision to pick Bombardier was made, after the vote or before it.