Ok I got a reply from them:
Was directed to this page instead of the one I found at first up top:
https://www.mymoto.com.tw/#/newdetal/1183
Told I can join VIP for 1500, and
“唯一的差別是 租車時要預刷信用卡7萬元 換車時退刷”
“The only difference is you have to pre-swipe 70000 NTD onto your credit card, which we’ll return when you bring it back”
I don’t know the SOP for moto rentals in Taiwan, but with how expensive they are here it seems kinda reasonable. But he said “the only difference” which implies that locals have a different procedure? In a good mood and hesitant to ask lol.