While it's true that acid rain is not caused by CO2 per se, it is generally caused by products of combustion. The context was specifically the other harmful effects of the root cause - burning fossil fuels - which a solar shield would not address. So it's wrong to narrowly focus on CO2.

As for ocean acidification, it's a pretty big deal: https://en.wikipedia.org/wiki/Ocean_acidification#Predicted_...

"Importantly, the rate of change in ocean acidification is much higher than in the geological past. This faster change prevents organisms from gradually adapting, and prevents climate cycle feedbacks from kicking in to mitigate ocean acidification. Ocean acidification is now on a path to reach lower pH levels than at any other point in the last 300 million years.[81][71] The rate of ocean acidification (i.e. the rate of change in pH value) is also estimated to be unprecedented over that same time scale.[82][14] These expected changes are considered unprecedented in the geological record.[83][84][85] In combination with other ocean biogeochemical changes, this drop in pH value could undermine the functioning of marine ecosystems and disrupt the provision of many goods and services associated with the ocean, beginning as early as 2100.[86]"

See also: https://en.wikipedia.org/wiki/Ocean_acidification#Impacts_on... https://en.wikipedia.org/wiki/Ocean_acidification#Other_impa... https://en.wikipedia.org/wiki/Ocean_acidification#Impacts_on...