Theoretically, I guess you can, as the work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license.
Anyway, for correctness, I think it’s better to drop a line to DigitalOcean just to ask for the moral permission and to tell where you’re going to use their work.

Have a nice day,
Alfio.

What @missaglialfio said I believe is the correct answer. I’m not sure where you’re just going to get the icons though, that aren’t already in a diagram. I’m sure DigitalOcean has all of them but I wouldn’t know where they’d be/if they were even available for use.

Because of the license you can modify them, but you’d have to give credit to DigitalOcean and not use it for commercial purposes.

What did you actually ask for?
I don’t see how they could forbid you to use those icons, as they license permits to share and adapt this work.
You should ask for more information with references to the legal terms.

@missaglialfio I couldn’t find anything about it on the @DigitalOcean Twitter, but I’d guess he asked for just the icons that weren’t already in a preassembled image, like I mentioned in my comment above.

The images themselves would be allowed for use, you’d be correct, but that doesn’t mean they’d put the assets (like just an individual server icon) up to the public.