I may be going out on a limb, but I have recently begn the process of updating from 1.5.0 to .1 (even though at any time 1.6.0 may get released), from the comparisons and the writeups out there, I would say that there is not enough changed that would prevent it's use. Realize however, in incorporating the changes any core files that are modified (or copies f in the case of templates) may be different than those of 1.5.1. Ultimately I would think the statement was made to prevent installation of the plug-in to older versions of Zen-Cart that would not (or may not) support the product as written.
That said, when doing the appropriate file comparisons care should be taken to understand what the modified does and why it is being changed. (Ie is it part of thhe plugin or there for some other reason.)
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