"I don't know, but I'll find out" is one of the most credible things you can say in front of a customer, and one of the hardest.
There's a fear in technical sales that any gap in your knowledge, exposed in the room, costs you the deal. So people bluff. A confident half-answer, a plausible-sounding generalisation, a smooth swerve. It feels safer than the silence of not knowing.
It's usually the more expensive choice. Technical buyers are, by trade, very good at telling the difference between someone who knows and someone who's performing knowing. The bluff rarely fools the one person in the room it most needs to, and the moment they catch it, every other confident thing you've said is now in question. You don't lose the one answer. You lose the credibility behind all of them.
"That's outside what I've worked with directly, let me come back to you properly" does the opposite. It says you know the edge of your own knowledge, which is exactly the thing that makes the rest of it trustworthy. Nobody is a complete expert in everything, and pretending otherwise is the tell of someone who might be pretending elsewhere too.
The trusted advisor isn't the one with an answer for everything. It's the one whose answers you can rely on, because they're honest about where the answers stop.
Confidence is knowing things. Credibility is being straight about what you don't.