Years ago I was one of the lucky ones to get a "survey audit", where they audit everything just to see what taxpayers are doing. It's not an indication of blame, but it covers everything.
At that audit, I was asked specifically the method I used to calculate mortgage interest divided between myself and someone else. I was a bit worried I'd done this wrong, and showed the auditor my spreadsheet for calculating the exact percentage interest based on overall contributions to the mortgage. That satisfied the agent, although I doubt an arbitrary method would have done so. To my mind, two people on a mortgage with only one person paying the mortgage sounds arbitrary.
I think seattlecyclone's interpretation of IRS publication 936 matches my experience more than that article.
Regarding the advice to try this 3 different ways, I can't find the bio of that author. Several websites do nothing but mention the author's name, with no links with more detailed information. It looks like an anonymous person giving advice, with no indication of their expertise (much like me, actually!).
But having personally been audited and having this exact issue come up, it's my impression you will need to show the IRS how you arrived at the split of mortgage interest.