Everyone thinking the lightning hid the date proposal as an accident, but that rumble just before she spoke, maybe she was expecting to have her words drowned out.
It felt like she was dragging out the syllables until the lightning, then she just blurted out the date idea during the lighting on purpose. If she really didn't want it to be hidden by the lightning, she'd have waited until after the lightning and probably not stretched out saying 'theeen' like that.
I mean, just saying it's a possibility based on the context. It could still have been an accident, but she seems so proud of herself afterwards, too.