Regardless of whether the managers are clueless or not, software companies operate on trust that the engineers' estimates of how long work will take, is based on the difficulty of the work with the natural assumption that you are devoting all your working hours to that work, and that there's not some hidden multiplier in your work estimates because you're splitting time with other jobs.

Unless you tell your team and your manager "I only work 1/x of my time at this job", it is deceit.