We have to be careful when talking about "supply" in this context. It can mean 2 things:
- the money supply (14 million BTC and adding 3600 / day)
- the mining production output (amount of 'commodity' the producers are willing to provide to the market)
In the second meaning, the categorization of the halving as a 'supply shock' is correct.
A "supply shock" is a sudden shift in the supply curve. Technically, tere is a supply shock every ten minutes when 25 bitcoins are added to the money supply. You could also say there is a daily supply shock relating to the addition of 3600 BTC. It depends on the time scale.
The halving is not a supply shock. It is a reduction in the severity of the ongoing supply shocks.