Read Bob's message again, carefully.
If you intend to have different articles and organization for phone and tablet, you'd want two top-levels.
If you're building different phone and tablet, pixel-perfect content, you'd want two top-levels.
If you want the same content and organization on both phone and tablet, one top-level will make things easier.
If your content is HTML-based with good CSS, it can adapt to both phone and tablet, and one top-level is great.
If your content is pixel-perfect and you have one top-level, the content will scale. So some things might be small, others large.
But browse pages can have BOTH phone and tablet designs, allowing them to look good on both types of devices.