I don't think that unmalted barley has zero DP. Doing a brief literature search I couldn't find any empirical evidence. However, diastatic power is correlated both positively and strongly with beta-amylase. Essentially DP is measuring beta-amylase but technically it is measuring the reducing power of all the enzymes. Brewing scientist also measure alpha-amylase activity on its own. If the DP measurement was an indicator of alpha-amylase activity then they wouldn't need to do this.So a malt with no diastatic power still has b-amylase but needs to be released by enzymes from the base malt?
Malts have both beta- and alpha-amylase, and other active enzymes generated in the malting process; whether they remain active depends on how it was kilned. Crystal malts and roasted malts are kilned at a high enough temperature for a long enough time that the enzymes get denatured, which means that they really don't have any diastatic power to speak of. Pale malts are kilned at a much lower temperature so that the enzymes are preserved. Pale Ale malt is kilned a little higher than Pilsner malt, but is also more fully modified (or at least that was the case in the past, modern Pilsner malt is often well-modified), hence it is a bit darker but still has a reasonable diastatic power (one of the reasons Maris Otter is so popular is because it has a much higher DP than most pale ale malts, making it effective with large amounts of specialty malts). Vienna malt is kilned at a slightly higher temperature than pale malt, and dry ('toasted'), which is why they lose a significant amount of DP (they still can convert themselves, but not as much adjuncts as pale malts). Munich malt (IIUC) is kilned at about the same temperature as Vienna malt, but 'wet' (steamed or 'stewed', like crystal malt), and for a longer time than Vienna malt, hence it loses still more DP; lighter Munich malts (10 SRM or lower) generally can self-convert, but darker ones often cannot.So a malt with no diastatic power still has b-amylase but needs to be released by enzymes from the base malt?