fedpkg srpm will download the file correctly.

Are you sure you are using the spectool script from this old, probably unmaintained repository instead of one of the other programs with the same name (which in many cases comes with the rpmdevtools package)? The script in this repository doesn't accept the -a argument at all.

But to try to be helpful:

Firstly, spectool doesn't download from the lookaside or interact with it in any way. fedpkg srpm and spectool do rather different things. spectool is intended to fetch the upstream sources when specified as URLs in the spec file.

I used both the spectool script in this repository and the one in rpmdevtools to download the sources using the current rawhide plasma-workspace spec and both of them work as expected; the one source file is downloaded correctly and without error output.

Maybe you could provide some indication of the errors you receive when running spectool. But since it seems likely that you aren't running the script from this repository, you really should figure out which one you are running so you can report the issue in the correct place. type spectool will tell you the path, and you can pass that to rpm -qf (on rpm-based distros, of course) to figure out which package owns the file.

now is part of rpmdev-tools /usr/bin/rpmdev-spectool