"Plausibly true" is very close to "likely true". "Plausibly true" describes something that does not contradict what is already known and agreed upon. Moreover, correlating positively with other facts makes the opinion in question even more plausible.
"Superfluously true" is not something one stumbles upon commonly: it is not a standard expression, or if you want, not a cliche. As such, if you choose to use it, you need to make sure that the context is very clear and helps determining the meaning you intend, instead of relying on it to do so. In as much as meaning is concerned, "superfluously true" may be closer to "seems to be true, but it is not really so", but it depends largely on how you use it (on the context).