@dclaar isPressed() reflects current state of the button (is it pressed?), and returns a Boolean. wasPressed() is an event trigger you use to execute code when the button is pressed.
Sample code below (generated from uiflow) to illustrate.
from m5stack import *
from m5ui import *
from uiflow import *
lcd.setRotation(1)
setScreenColor(0x111111)
isExit = None
def buttonB_wasPressed():
global isExit
isExit = True
pass
btnB.wasPressed(buttonB_wasPressed)
setScreenColor(0x000000)
axp.setLcdBrightness(40)
isExit = False
while isExit == False:
lcd.print((btnA.isPressed()), 3, 0, 0xffffff)