Making any investment yearly at the same time is attempting to time the market, it’s a bet that the market will be lowest at that point vs the rest of the year. Otherwise, why pick Jan 1? Why not pick July 4? If the price is lower on Jul 4, you end up with more shares, as well as a small increase due to 6 months of interest.
When you DCA, you basically admit that you don’t know how prices are going to move, and you are spreading out your risk. Yes, DCA over 12 months may leave you with slightly less than if you put it all in on Jan 1, assuming the price was the lowest on Jan 1. But if you have monthly investments that whole time, it’s likely that at least one or two of those might have been bought at a lower price than Jan, and it may turn out DCA could result in more shares of whatever you are buying.
The “time in the market” adage applies over years, not months. On a scale of 10 years+, it doesn’t really matter whether you bought in Jan or July.