Yes it does.Does it? I see that it shows that the Core rules sold best. That the sales started with a large number and then had a large drop off. That campaign settings sold closer to the Core rules sold better than those sold later. That rule releases sold better overall, and gave a bump to other products.
That data doesn't say that sales from Dark sun was what made Dragonlance sales decrease, or that releasing Forgotten Realms made Dragonlance decrease.
Instead it shows a distinct pattern of strong sales at the start which then have a drop off.
Nothing indicates that they were cannibalizing their own sales with other lines.
WHAT DID happen (not sure if the book covers it, probably does if Riggs is as thorough as people say it is) is really dumb financial decisions.
When you make a compilation of magic stuff in a book which no one really wants, and start off with a sale price of maybe $15 you might sale a few and perhaps turn a profit. Make it $20 and you will sell less. Then when the boss comes and says...put leather covers on it so that it will cost $50 to make...you just have to scratch your head because that book is NOW going to LOOSE money no matter how you try to sell it. You can't sell enough to make back the money if you sell it at a high enough price point, but if you don't sell it that high, you lose money with each book. It's a lose/lose situation (edit: Example of a situation that may or may not be based on an actual situation that occurred).
It's not that the book CAN'T make money, but the decisions behind how it is going to be published makes it a lose/lose situation. The books COULD make money (and WotC showed that very well with how they "split" the lines at some points, though probably not all the time, at one point in particular it probably had some harsh realities that came to fruition, but not the time to cover that), but decisions were made that were terrible choices.
Investing money into sidelines that had nothing to do with the products being made, relying overly on future sales as hard numbers rather than estimates, counting on borrowed money as income, and many other BAD financial decisions are what sunk TSR most, not the multiple campaign settings themselves. Those are things that I don't recall being mentioned when WotC was trying to advertise 3e, but the reality (boring as it probably would be to most who would read such stuff) is probably the harsher truth. It was really bad financial decisions in regards to money that sunk TSR.
Making three complicated expensive products for the same money vs making one or two costs you more to make and thus squanders your profits.