Tommy Shannon is indeed famous in blues music circles. And let's stop for a moment and consider - the Blues is one of the very few music forms created in and native to, the United States (...it was derived from African-American music but is a distinct form.) Therefore, the reputation of a blues artist within the US is more pertinent to their status than their reputation in other countries.
The Online Dictionary defines "legendary" thus:
a. Celebrated in legend.
b. Extremely well known; famous or renowned.
By the second definition, one can make a good argument that Tommy Shannon has indeed achieved legendary status in the US...and that's where it counts.
With all due respect to Australia, that country surely has its own "legendary" musicians that we've never heard of in the US. When it comes to blues musicians, IMO their US reputation is most pertinent...and their reputation in Australia not very pertinent.
Let's face it, most of the world's population doesn't know who Robert Johnson was, but anyone who has ever listened to any amount of the blues does. I consider him legendary - and not only is he famed for his music, there are legendary tales told about him.
I can accept that. You stated your case well.
As for famous Aussies, there are a lot and in many many fields, considering there are only about 9 million adults in the country but in reality, most only became famous when they left the joint. Fame in the US brings financial security. Fame in Oz usually means you still need a government pension to retire on. The country is simply too small to benefit from.