So I'm supposed to check an outdated article quoting an outdated rule book that in no way is considered an official interpretation (yes, even published on the PDGA website, it is not considered official)? Oookay.
Unless and until the PDGA Rules Committee either disavows or revises the interpretation in the Rules School article (which is entirely consistent with the RC's principle that in the absence of compelling evidence that a violation has occurred, the presumption for purposes of rules application is that no violation has occurred), the interpretation in the article continues to be the precedent by which the matter is to be adjudicated.
Rules Q&A 14 and 34 are particularly relevant to the presenting issue, in that both specifically address situations where the finish of a disc's flight is not visible to the thrower's group, and articulate the principle to be used to determine how to proceed in analogous situations.
QA 14: Lost or OB?
Q: My throw was headed toward an OB lake when it went out of sight, and we never found it. Do I play it as lost, or as OB?
A: If your group agrees that there is compelling evidence that the disc went into the OB lake, then you assume that that is what happened, and play it as OB. If there is uncertainty about whether it went in the lake, then you play it as lost. Applicable Rules: 804.04 Out-of-Bounds.
So, absent of "compelling evidence" that the disc went into the lake, the presumption is that it did not.
QA 34: Putts not Entering the Target Properly
Q: Everyone in my group watched my soft putter push thru the side of the basket and land completely inside of it, not wedged at all. They said the putt was no good. Are they right?
A: Starting with the 2011 rules, throws that are observed by the group or an official to enter the target by wedging through the tray or by dropping through the top of the chain support will not be considered good, even if they come to rest in the basket or chains. If no one sees the throw on a blind hole or when the target is too far away, the benefit of the doubt is given to the player. [emphasis mine] Applicable Rules: 802.05 Holing Out.
So, absent observation that the disc did not enter the target legally, the presumption is that it did enter the target legally.
The operative principle in both these interpretations (and in the Rules School article) is that certainty, either in the form of "compelling evidence" in the case of QA14 or observation in the case of QA34) that a violation has occurred is required to penalize the thrower: that is, the RC assigns the burden of proof to showing that a violation HAS occurred, not that a violation has not occurred. If there is uncertainty that a violation occurred, benefit of doubt is granted to the thrower.
Logical extension of this principle (801.01.A) to the question in the presenting scenario, "Did the disc touch IB before coming to rest OB?" places the burden of proof squarely on the claim that the disc did NOT touch in bounds. In other words, the requirement is not to prove that the disc
didn't touch in bounds, rather than to prove that it did.
Assuming the bank is in bounds (a reasonable inference, given that the OP's statement that "the flight of the disc would leave you to belive that it hit the bank and then went in the water" would be unnecessary and irrelevant if the bank were OB), the argument that the disc did not touch IB rests entirely on: (a) the fact that it was not observed to touch IB; and (b) fact that it came to rest OB, neither of which rise to the level of "compelling evidence."
(B) may be disposed of by reference to the reasonably common experience of seeing a disc land on an inclined surface and skip or roll away: sometimes quite a distance away (see Paul McBeth's drive on the final hole of the European Masters).
Against (a) is the non-thrower's statement that "the flight of the disc would leave you to believe that it hit the bank and then went in the water." While not conclusive or compelling evidence that it
did hit the bank, it provides sufficient warrant for conceding that it
may have done so.
Consequently, the absence of "compelling evidence" that the disc did not hit the bank, coupled with the affirmation that the flight of the disc would lead one to believe that it did, provide sufficient ground for proceeding on the assumption that the disc did hit the bank.