They could well have spoken to SP's lawyer but just refused to be a witness or contribute evidence to the case. It'll differ between each person but often when you're gathering evidence, you get intel which you can't rely on but which can give you more confidence in your position & claims - in this case, that SP isn't alone in her position & stating that isn't misleading. They just can't necessarily back that with signed statements & people willing to testify to help the case.
This can happen a lot in litigation - you get details of something someone said or heard & you try to narrow it down to identify the person & approach them to see if they're willing to give a statement. Some people will speak but only verbally & you can't really rely on that as evidence.
My take, which could still be wrong & you could be right in saying it's not a strong claim, is that the solicitor stating this has something more than rumour or hearsay, only it's either not necessary to the case to push for them to be a witness, or the person/ has/have said they won't give evidence in court.
Looking at the fact IB is the DEI Lead, is involved in work on EQIAs as part of her role & also that there is no EQIA in support of her advice to SP's manager about DU being entitled to use the female CR, I'd say that those 20 other people who either felt uncomfortable or objected to DU being given permission to use the female CR aren't needed to challenge IB, or her manager, on what steps they took to assess the impact of allowing a male doctor who claims to be a woman to use the CR, on female staff. They have nothing. I'm fairly confident of that fact because these policies aren't implemented because they've actually researched the impact & sought women's views on this stuff. It's all imposed without any dissent being permitted. With the threat of disciplinary measures should anyone speak up.
Again, this is all my guesswork/speculation & you could be right that it's a meaningless claim or not back up strongly. My view is, it doesn't matter & it's not something NHS Fife have anything they can rely upon to counter in any way.