Is this to say that it may have them lowered,
Yes and no: I think it was originally intended to
allow it to get the values lowered, if something would cause that. At the same time the text doesn't says explicitly, nor should it maybe, since
it follows logically that it's possible.
Problem with writing "Half-tree may not get its combatstats made greater in any way. It may still get them lowered." instead is that we not only cover the special rule for the card that negates the core rules, we also explicitly explain a situation which
doesn't negate or change any rule... meaning, it doesn't really need explaining. Also, while we're at it, and if we accept that, we should/could explain a couple of others that also don't need explaining, like for example: "Half-tree may not get its combatstats made greater in any way, by equipment or spells. It may still get them lowered by something. Half-tree can still die if correct creature type deals damage to it."
"Effects that would alter Half-tree's combat stats have no affect", maybe?
I think that is slightly more straight forward, but it will also become even harder to kill off Arknym if you lack the proper creature type to deal damage to it. Like it is now the opponent could try to lower your DEF with a spell or instante etc, and then Arknym would still die, even if you don't happen to have a Caster, Legend or Leader in your deck.