The difference is intent. If she admitted and was open specifically what this came from and why she did it (which by all accounts I can find she did) and there was no attempt to profit from it (which I can't find she did) then I wouldn't call it a rip-off. It would be like me paraphrasing a great statement you made and giving you credit. The intent was not to rip off your statement but to honor or study it.
If someone can show me that she purposefully did this to deceive and tried to hide the source and she attempted or did profit from it, then we can talk about it being a rip-off.
I think you have it backwards...I’d rather see a statement around 1963, in her own words, not someone else’s, where she says it was a “study”.
To my knowledge - a study or copy of another artist's painting is not renamed & it is explained right on the copy of the painting - not explained somewhere else. One should not have to search around to identify it as a copy.
The proper signature would read: By Alma Thomas after The Snail (L'escargot), by Henri Matisse