Also, Blender is not a grocery list app. While disregarding novice users does result in an application that's impossible to learn and that's obviously bad, it's equally unproductive to optimize an interface for people who see it for the first time. Blender is the kind of application that you spend tens, if not hundreds of hours learning before using it productively (let alone professionally). That's the target audience you're designing for, not people who download it and uninstall it if they get bored in the first thirty seconds.
Plenty of professional tools (Photoshop, Final Cut, Figma) have monochrome icons without it being a usability issue. These are all content creation tools, including Blender. The UI and content should not be fighting for attention. It's clear which of those two should primarily be on display.
I think it's entirely possible that Blender just has poor icons - but it seems demonstrably false that monochrome icons are inherently inaccessible.
My best guess is that Blenders real issue is a lack of structure and clear grouping. Providing the icons are in a logical place, it's easy enough to find the correct one, without them needing to be visually distinct on literally every dimension. But if there's no logic to the placement and the icon you're looking for could be one of thirty, then I agree that's an usability issue - just not that the icons are at fault.