Of course it wasn't, Stilicho was simply trying to keep the Empire together and fell out of favor because of his methods (such as seeking an agreement with Alaric to help the WRE militarily) Constantius's acclamation wasn't an admission of defeat, it was a recognition of his powerful status in the Empire, if you ask me, it's pretty suspicious that he died after only seven months, but even if Honorius hadn't made him Emperor, i doubt Constantius would rebel against the man who, despite being a weak ruler, has survived several usurpations and has shown himself willing to kill members of his own family to protect his throne, (Stilicho was Honorius's father in law and Honorius could hold Constantius's son Valentinian III as a hostage for his loyalty) and from what we know, Constantius probably wasn't interested in the Imperial throne since he complained about how burdensome and restrictive the position was because let's not forget, the position of Emperor in this period is one where power is nominal and threats are predominant, Generals such as Castinus, Aetius, Ricimer, and Orestes all had a chance to become Emperor and didn't take it, they chose to remain in their positions as Magister Militum since that position was more powerful and influential.