Great article! I think the explanation of leaf vs interior pages might have benefitted from a diagram. My understanding after reading it is this:

                        3───────┐
                        │pk data│
            1───────┐ ┌─┤A  ....│
            │rng ptr│ │ │B  ....│
          ┌─┤A-B 3  ├─┤ 4───────┤
          │ │C-D 4  │ │ │pk data│
0───────┐ │ └───────┘ └─┤C  ....│
│rng ptr│ │             │D  ....│
│A-D 1  ├─┤             5───────┤
│E-H 2  │ │             │pk data│
└───────┘ │ 2───────┐ ┌─┤E  ....│
          │ │rng ptr│ │ │F  ....│
          └─┤E-F 5  ├─┤ 6───────┤
            │G-H 6  │ │ │pk data│
            └───────┘ └─┤G  ....│
                        │H  ....│
                        └───────┘

(also reading https://www.sqlite.org/fileformat.html says that the row’s rowid will be used as the B+Tree page key instead of the PK, but I assume that’s a simplification on the article’s part because an integer PK will be used as the rowid. So assume the PKs in the above diagram are octodecimal :-) )