Bazel C++ rules(cc_liibrary): why can a target access headers of its dependency that are not exposed via hdrs?

Bazel simply does not implement any header privacy checking by default. However, under clang and a recent version of Bazel, the so-called "layering check" validation can be enabled with the layering_check feature. For instance, with the OP's example repository and Bazel 4.0.0, I see:

$ CC=clang bazel build //sub --features layering_check
INFO: Analyzed target //sub:sub (16 packages loaded, 56 targets configured).
INFO: Found 1 target...
ERROR: sub/BUILD:3:10: Compiling sub/greet.cpp failed: (Exit 1): clang failed: error executing command /usr/bin/clang -U_FORTIFY_SOURCE -fstack-protector -Wall -Wthread-safety -Wself-assign -fcolor-diagnostics -fno-omit-frame-pointer '-std=c++0x' -MD -MF ... (remaining 28 argument(s) skipped)

Use --sandbox_debug to see verbose messages from the sandbox clang failed: error executing command /usr/bin/clang -U_FORTIFY_SOURCE -fstack-protector -Wall -Wthread-safety -Wself-assign -fcolor-diagnostics -fno-omit-frame-pointer '-std=c++0x' -MD -MF ... (remaining 28 argument(s) skipped)

Use --sandbox_debug to see verbose messages from the sandbox
sub/greet.cpp:1:10: error: use of private header from outside its module: 'f.h' [-Wprivate-header]
#include "f.h"
         ^

layering_check may also be enabled on a per-target basis by setting features = ['layering_check'] in the BUILD file.