Pico doesn't require the PHP extensions itself, but erusev/parsedown-extra does. The explicit composer.json requirements are necessary until erusev/parsedown-extra#85 gets merged. Both extensions aren't part of Ubuntu's default LAMP setup anymore.
The comments of the "instance Pico" section violate PSR-2, but we want to keep them anyway
See https://github.com/picocms/Pico/issues/285#issuecomment-160130798