The text says to minimise electrical noise, use a battery--good advice--but the the illustration shows a power bank, which almost certainly has a SMPS between its internal battery and its output USB port(s). Quite likely a noisy SMPS.

Use a low drop-out linear regulator to supply the 3.3V the Pi requires from a 4.2V LiPo or four 1.2V NiMH cells.