The implementation of webdriverIo is slightly different from selenium.
In webdriverIO $('table').$$('td')
will check for direct child 'td under 'table', and as td is not a direct child it won't return anything.
instead, try:
$('table').$$(' td') (See if this works)
or
$$('table td')
Space in 'CSS' is similar to // in xpath
Better raise issue with webdriver io