The power switch was dead as a doornail (whatever a doornail is?).
The idiom 'dead as a doornail' dates from the fourteenth century. Costly metal nails hammered into the outer doors of the wealthy (most people used the much cheaper wooden pegs), which were clinched on the inside of the door and therefore were "dead," that is, could not be used again.
And as you've now learned Canon's claim about the caps is correct. This can happen with anything using a voltage reducing/regulating power supply. Computers and printers typically run at a low internal voltage of anywhere from 12-48 volts. The caps store power to prevent brown-outs and to absorb overages from voltage spikes, thus reducing and regulating.
Using an automatic voltage regulating (AVR) UPS with a printer is just as important as using one for your computer these days.