Huh, Nel is making a good point here. Every single person who heard something on that night, woke their partner other one up to discuss it with them. The only ones who didn't were those who had no partners, or those where the partner was already awake and they discussed it.
When he heard something when he was with Taylor, he woke her up to discuss it.
This night, he says he told her something, but he didn't discuss - he didn't wait for a response. Was he loud? Was he whispering?
Either way, he didn't discuss it with her, and this is unusual behaviour.
He also pushes the point that he did not wait for a response.
The whisper/low tone was more important to OP than it was to Nel. OP says unprompted that it wasn't a whisper but a low tone. If he'd whispered, the indication is that Reeva must have been close enough to hear a whisper - but she also needed to be far enough away so he couldn't have seen her.