Others can comment as well given I am no expert but where does it say work continuously to prove physical presence. Does a stay at home mother get disqualified for citizenship due to no taxable income but has not left the country ever? This is more about if anyone worked that they have met their obligations as far as taxes are concerned.
Even working continuously in Canada does not mean someone has not been out of the country for work or vacation which needs to be deducted from physical presence.
Requirements related to income tax
The applicant must have met any applicable
requirement under the Income Tax Act to file an income tax return for three taxation years that are fully or partially within the five years immediately before the date of application.
If you read the info in the above link there are exceptions made where income did not met the minimum income threshold to file a return.