Your brain does not multitask, it switches, and every switch leaves a residue of attention behind that eats into your cognitive capacity. That is where uninterrupted blocks earn their keep: inside such a block you work on one thing, with no notifications and nothing on the side.
A block like that has a shelf life. After a stretch of sustained attention your sharpness drains away on its own, and a workday without breaks, an open-plan office and a chat that never falls silent hold you in that state of low attention and constant stress. Pushing on feels productive at that moment, while the quality of your thinking has already dropped.
So blocks and breaks do not contradict each other, they work on two timescales: no switching within the block, a reset between blocks. In practice that means stepping away briefly every 60 to 90 minutes without new input, plus a clear stopping time at the end of the day, because it is in those quiet moments that your brain consolidates and creativity comes back. What wrecks your attention is the task switch, not the break.