CMakeLists.txt currently states:

However that isn't always true. If, for example, depends is built with DEBUG=1, and CMake is configured with -DCMAKE_BUILD_TYPE=Debug, -O1 will actually be used. This is confusing, not just because the documentation is incorrect, but when making a change to switch the build type to Debug in oss-fuzz, it's caused unexpected results: google/oss-fuzz#12443 (comment).

Rather than assuming that builds with the Debug build type don't use optimisations, the check should be fixed to actually check what optimisation level is being used.

i.e:

make -C depends/ NO_QT=1 NO_WALLET=1 NO_ZMQ=1 NO_UPNP=1 -j17 DEBUG=1
cmake -B build --toolchain=/root/ci_scratch/depends/x86_64-pc-linux-gnu/toolchain.cmake -DCMAKE_BUILD_TYPE=Debug
<snip>
C++ compiler flags .................... -pipe -std=c++20 -O0 -ftrapv -O1 -g3

(-O1 is last, so is what will be used).