The word نحر as a noun originally refers to the upper chest or neck.
The word نحر as a verb refers to "correctly [finding] the upper chest." It is commonly used in the context of finding the animal's neck for sacrifice. So, due to that, the word has commonly come to mean "sacrifice."
In this verse, انحر is the command form of the verb.
Since, the primary signification and usage of the word is for "sacrifice," that is how the vast majority of the Salaf interpreted it as.
However, the word in itself could be taken in other ways that refer to the chest in less primary meanings.
Some minor opinions pretended thus that انحر refers to a part of the prayer. However, they differed on what exactly.
A few different opinions:
It means to correctly place the hands (right on top of left) on the chest in prayer.
It means to raise the hands when you do takbir in the beginning of Salah to the height of the chest.
It means to face your chest (hence your whole body) towards the qiblah.
As for their translation of "moderate," that's just a bad translation choice. It doesn't even fit with the very opinions they present. What they meant to refer to is to "stand upright (or moderate) in Salah such that your chest and neck are straight."
It does not mean "moderate" in the normal sense of the word even in their opinion.
As for some reasons why the vast majority chose the interpretation of sacrificing an animal:
That is the primary signification of the word. These other interpretations require a bit of squirming since they're not primary.
The normal pattern in the Quran is for Allah to command prayer then command Zakah (charity). Sacrifice is a form of charity, since when you sacrifice an animal you give much of it to charity. So, it makes more sense for it to be sacrifice here.
It does not seem to make sense for Allah to command prayer then command a part of prayer again.
The verse is meant to indicate exclusivity to Allah. It means: Even if the idolators pray and sacrifice to their idols, you should only pray and sacrifice to your Lord alone. This meaning is lost if we understand the word as referring to a part of prayer.
That is why you will find the majority if not all translations translate it as "sacrifice."
I do not think you should consider finding one obscure tafsir of the verse as that being the standard Shia opinion. I suspect the normal Shia interpretation would also be "sacrifice."
