Tieren are Zaku. There, I’ve said it.
If someone who is unfamiliar with Gundam, specifically Gundam OO, and they say something along the lines of “I like the new Zaku designs in OO”, what they mean is they like the Tieren design. Because Tieren are Zaku.
Tieren are NOT “not Zaku”. They are not so radically different as to be totally indistinguishable or exclusive from Zaku. They might not be identical to the old designs, but there’re enough design cues to know from where they originated from.
That is all.