The article is wrong, AGU hasn't been used for LEA on mainstream Intel CPUs in a decade at least.

LEAs are calculated on ALU execution units like other ALU ops. r13 and rbp are special because the usual way of encoding those registers in an indexed addressing mode without an offset was repurposed instead for another mode: offset only or RIP-relative in x86-64.

This affected only ebp in x86-32 but also r13 in x86-64 because of the way the instruction was encoded.

As a result, if you want to use rbp as a base, you need to use an explicit offset (which can be zero) and that ends of triggering the "three source rule" for slow LEA in the case of indexed addressing, even though you have included any offset in the assembly.