Einstein's work was linked to the 'creative' ventures he had later on. From Wikipedia: "Much of his work at the patent office related to questions about transmission of electric signals and electrical-mechanical synchronization of time, two technical problems that show up conspicuously in the thought experiments that eventually led Einstein to his radical conclusions about the nature of light and the fundamental connection between space and time." So really his office job was promoting his creative thoughts.
Bukowski (holy cow, what a diametrically opposed example!) is not so much a creative person as a person with PTSD. What's more, he wasn't writing when he was working for the post office initially, (and incidentally he was a letter carrier, meaning being outside). He left after 3 years, but returned a decade later after a divorce (and her subsequent death). Once he was back on the job, an old flame's death prompted more writing, and over the next decade he continued to write, though one has to wonder if it was because of his office gig or in spite of it. At the end of the decade he quit so he could write full-time, which to me says that he both disliked his job and also couldn't be completely creative while working there.
So while Einstein was directly inspired by the subject matter he reviewed at his job, Bukowski was driven by more personal motivations, along with perhaps a disgust with authority and human nature in general. Hard to say those are good anecdotals to defend the idea of general creative work being at all easy while working in a typical office job.