The weather conditions – all over the central and southern were NOT sudden and unexpected. Severe thunderstorms had, in fact, been forecasted (quite accurately as it turned out) by the NWS early in the week for Friday through Monday, with emphasis on Saturday and Sunday.
I didn't look at the forecast as far south as Yosemite- but the forecast for Tahoe that I looked at Sunday morning before heading out was only showing 20% chance of isolated storms.
At least for the Northern Sierra, the bulk of that system came through on Friday/Saturday. (And apparently Sunday's storms didn't reach as far north as Carson Pass)
And I had a pretty good vantage point at 11k at ~10am and there was barely a cloud in the sky on the Sierra Crest.
The cell that formed over the Sweetwaters formed organically over the peak.
I think you are correct that this wasn't a system that came completely out of the blue- but it also wasn't necessarily the case that the climbers ascended HD during active storm conditions.
Conditions were marginal, unfortunately the hiker gambled and lost.