The old saying is specific to Black Locust, as for as I know, and an exagge rated compliment to its longevity as fence post material.... and, in effect , seconding Karl's opinion/fact of BL being the best wood for posts.
Additionally, getting away from helping the OP: In some cases, catalpa makes for long lasting fence posts, despite the wood being soft and porous. Many split rail posts take root and grow into tree s, hence, lasts a long time.
Our old homestead had a fence line, as this, until Hurricane Gustav blew do wn most of the trees. I had some of the upper trunks milled and the sawdus t or unseen chemistry, when working the subsequent boards, was very irritat ing, even when slightly inhaled. That was some potent stuff, when sawing, working with it. The wood is a pretty, chocolate brown. Here's a small be nch from that catalpa cache -
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Sonny