The fact that an added commandment is present is trouble enough, and to worship on high places elsewhere noted in the Bible, would not fit it being original. It seems more a post-fact rationalisation by the Samaritans of their Gerizim worship.
There are more substantive changes than those presented in this article. In the Masoretic text version of Joshua, the mount of blessing is Ebla and cursing Gerizim. The Samaritan one is the other way round, although this techmically is not the Torah per se. Fruitful Gerizim makes more sense in this regard, than barren Ebla, but the Samaritan version makes explicit that this is the Moriah where Abraham almost sacrificed Isaac. To support this, their Torah reads Amoriah there, which fits it as being in the land of the Amorites and therefore Gerizim. Interestingly, the LXX also uses Amoriah, and the fact that Genesis has a character Hamor from Shechem is further support for this. This is a good example of how 'minor spelling changes' as it is called in the article, can often have substantial implications. For this extra letter is striking at the root of the Jerusalem Temple's primacy as the site of Abraham's sacrifice.
Additional narrative can have the same implication. Look how often verses are taken out of context or used in conjunction with other passages.
If these are ancient interpolations, then they were omitted by the elders who compiled the LXX and by the Masoretes, which likely means that they are inherently pro-Samaritan or incongruent to the way the Torah was understood by them.
The Samaritan Torah we have today is not an ancient written text that was preserved like the Qumran scrolls, but copied texts whose ultimate version we read today. This was done in a narrow geographic area and we can only compare it to our extent other Torahs. As the Qumran scrolls attest, it is its own ancient tradition and a version of it was present in the first century, but to assume its legitimacy is a bit much. All the various versions of the Torah should be read in conjunction to decide this and the fact that the MT and LXX are so much more akin to one another than the Samaritan, should give one pause. The Christian OT is the former depending on denomination, not the latter after all.
