To begin with, I tend to disagree that the alternative the author suggested ("yīqǐ Zhīfùbǎo 一起支付宝") is catchier. Using "let's" in this context does forgo rhythm but it's also a lot catchier when it's read out in an advertisement for exactly the same reason—I would argue that people remember it because of the sudden interruption introduce by the disruption in rhythm and change in language.
In fact, if this multiscriptal advertisement was carefully designed (instead of the designer thinking that it's simply cool), I would go as far as arguing that they only want people to remember the second half of the phrase, "let's 支付宝".
As for the conclusion that "[Alipay] wanted to sound international, since Alipay has global aspirations", as I mentioned earlier, I don't think it's at all surprising.
Growing up in Hong Kong, multiscriptalism in advertisements has been around for as long as I have been alive and the purpose, at least for as long as I have been able to understand the purpose of advertisements, has always been to either make a product appear to be hip and cool and/or international.