There is some truth to it, but it is not set in stone. Lawyers tell applicants this to ensure they won't travel outside to minimize risk of being denied re-entry, just to be on the safe side. BUT this is dependent on your specific case. If you have a multiple entry visa/TRV which is still valid, you will be allowed to reenter regardless of the situation of your sponsorship application. If you are from a visa-exempt country, or a country that only requires an eTA, you can easily re-enter. If you were already working or studying in Canada on a work/study permit along with a TRV in your passport before you applied for sponsorship, and your permit and TRV are still valid, you can re-enter Canada. This was my case, I have left and re-entered successfully as I still have a valid study permit + TRV till 2019.Interesting. At the bottom of the page is:
Date Modified:
2017-04-18
When we applied in February,we were told and I know we also read that we could not leave as it would cancel my application
If you came on a visitor visa and your 6 months has elapsed and you are now on implied status waiting for you OWP, or have gotten your OWP, then it is riskier to leave and re-enter. Note that a permit (study or work/OWP) does not guarantee re-entry! It must be accompanied by a TRV (or visa-exemption or eTA).
Whether you can enter or not is up to the discretion of the CBSA officer and they follow the rules in their operating manual "Port of entry examinations", which you can access here and find your specific case: http://www.cic.gc.ca/english/resources/manuals/enf/enf04-eng.pdf
What you mentioned is cautioned on the full/detailed sponsorship guide on the IRCC website (http://www.cic.gc.ca/english/information/applications/guides/5289ETOC.asp#after). See below: