I disagree with that for thumbs, mine tend to be much flatter against the keyboard than my arched fingers, so I can hit the split space and mods easier with my thumbs if that row is at least 1.5u and preferably 2u for at least the split space keys. 1Us on the bottom row forces me to either contort my thumb or even worse, use my fingers for the bottom row.
I disagree with that for thumbs, mine tend to be much flatter against the keyboard than my arched fingers, so I can hit the split space and mods easier with my thumbs if that row is at least 1.5u and preferably 2u for at least the split space keys. 1Us on the bottom row forces me to either contort my thumb or even worse, use my fingers for the bottom row.