Question

Why doesn’t this work?

find . -maxdepth 1 -type f -print0 | xargs -0 .

All I get is xargs: .: Permission denied.

Answer 1

When you run . file, you invoke a the shell builtin . . Your xargs variant tries to execute the current directory.
Even if that did invoke the builtin, that command runs in a subshell, so all the “sourcing” would be useless.

Use shell globbing and a loop for that:

for file in * ; do
  if [ -f "$file" ] ; then
    . "$file"
  fi
done