What he proposes here is a kind of "fallthrough", like for branches of switches without break in C/C++, but here he wants to allow this also for if...end statements. "elseif" implicitly includes an implicit "break" before a new branch, i.e. exclusive conditions; and he wants inclusive conditions (which has very limited uses, just like break-less branches of switches that fall through.
However it has some usage (e.g. when compiling finite states automatas, or for "exit" procedures at end of blocks to perform cleanup of conditionally-allocated resources in backward direction).