The package is a customized one for DB410C and
apt-get source is fetching the default Debian one. To me it looks like there is a bug in
/etc/apt/sources.list.d/linaro.list (or in the release notes… or both).
The following lines are missing from
deb-src http://obs.linaro.org/linaro-overlay-buster/buster/ ./
deb-src http://obs.linaro.org/qcom/buster/ ./
With these lines added then the
apt-get source command should work correctly (note that is must be run as the linaro user, not as root).