現地戦士は古の巨人として伝わっていて各地に隠れ住んでいる、とか
(https://twitter.com/isiyumi/status/1148206756863942656)

これは中々完璧な解釈ですよ！？凄い！(漠然と勢いで描いてるせいで自分の中でも正解がない)
(https://twitter.com/isiyumi/status/1148428142169583617)
(said in response to https://twitter.com/nakurubox/status/1148252585075204096)

あえて付け加えるなら戦車パーツ(チャリオットベース)は下半身まで担当するイメージ。つまり後半に登場するメカと入れ換えることで人馬型になるという寸法
(https://twitter.com/isiyumi/status/1148428876424499201)

The native Warriors of this world were scattered in various places by the Ancient Giants, where they dwell hidden... or something.
(https://twitter.com/isiyumi/status/1148206756863942656)

This is pretty much a perfect explanation! Amazing! (Admittedly I sort of drew the whole thing vaguely, so I myself don't know the real answer either.)
(https://twitter.com/isiyumi/status/1148428142169583617)
(said in response to https://twitter.com/nakurubox/status/1148252585075204096)

If I were to further add to this... the idea is that the chariot part (the Chariot Base) is in charge of up to the lower body. So the plan is to swap things out for another mecha in the second half of the series to get a centaur form.
(https://twitter.com/isiyumi/status/1148428876424499201)