If I’m using a high diastatic malt, was it still necessary to add some acid malt into the mash?

Using acidulated malt is never necessary per sé, unless you want to stick to the medieval German Reinheitsgebot. Lactic or phosporic acids are frequently used to correct the pH of the mash (especially when brewing light beer styles with high alkalinity water) but acid additions are prohibited by the Reinheitsgebot, which is why German brewers had to create a loophole in the regulations and came up with acidulated malt. An appropriate dose of lactic acid or phosphoric acid does exactly the same thing. Phosphoric acid is better than lactic because its flavour disappears into the malt phosphates and leaves no residual taste (unless you acidify too much and your beer becomes too sour, of course).

As other respondents above have already pointed out, mash pH correction has nothing to do with the malt's diastatic power (i.e. with its capacity to convert starches into sugars).

Diastatic power refers to the enzymatic power of the malt itself – its ability to break down starches into even simpler fermentable sugars during the mashing process, but that's not related in any way on needing acid malt or not.

Acid malt can be used to lower pH as Philippe said, but depending on the recipe you will not need it at all, like on beers with lots of dark and roasted malts that usually lower the pH by themselves.

I usually do the pH correction with lactic acid instead of acid malt, so I mix all the malt into the water and wait 5 minutes before getting a pH read with the pHmeter.

It depends on the purpose of the Acid Malt in the recipe.

Not only it has high diastatic power, this malt also affect pH of the mash, so depending on the water pH, this malt could be useful... According to : https://beerandbrewing.com