Is a COM-Domain enough to secure a trademark?

The information you are given is incorrect. In the US you must be using the mark in commerce in order to get even common law trademark protection. You can also file for a state or federal registration-- for a federal registration you need use of the mark in interstate commerce (for a state registration you just need use within that state). You are in Canada so the use rules may be slightly different but I doubt excessively so. Canada also has common law rights in unregistered marks as well as the ability to file for a registration. There is more information here about trademarks in Canada: http://www.ic.gc.ca/eic/site/cipointernet-internetopic.nsf/eng/h_wr02360.html

From a business perspective it's always a good idea to secure the website for your brand as quickly as possible! But just a mere landing page is not enough to show use in commerce, you must be actively selling your goods and/or services (either on the page or elsewhere) in order to have the website count as use in commerce.