To be a "legend" you need to achieve some rare feats that few, or none, have done. It also has to do with time passed, as a young guy who does a great thing is naturally thought to have bigger things ahead of him that, then, define his career more.
It's silly to call Shogun a legend at 23 after winning the Pride GP, when most would agree his run had barely started. It's another thing now, also because he won UFC gold. He's the only one ever to win a divisional UFC belt and win a Pride GP.
Beating Monson & AA for UFC title defenses aren't such rare achievements, thus time passed grows Sylvia's legend with some added hyperbole, not that those wins have great historical context. They weren't the best wins then, hence they shouldn't be seen as such now.