After tinkering and re-rereading documentation I found another header to pass to wikimedia which does change the output, confirming that my headers are being received correctly.

wikiheaders=Dict("User-Agent"=>"learning_julia_test(;)julia1_11","Accept-Encoding"=>"gzip")# for testing

When I pass "Accept Encoding" : "gzip" the file that is returned is in a gzip format which confirms wikimedia is correctly interpretting what I send.

Thank you @C3roe for what I consider to be a better and more universal approach of using a service like requestcatcher.com (which appears to be owned by a company called Omni) to test if my code was behaving as expected and not depending on the intended recepient for confirmation. In cases where each api get request might have a significant cost or a consequence, this would be be much more preferable.