The ambiguity come from "next" has the meaning of "up coming"
The Seinfeld joke applied the "up coming" meaning of "next" to create confusion, but in practice, people always refer "next Wednesday" to "next week's Wednesday". If you meant to say "the up coming Wednesday" you would just say so -- "the up coming Wednesday"
It is the same in Chinese, the term "下星期三" always refers to "下周的星期三" ; if you meant to say "the up coming Wednesday", you have to say "即将到来的星期三" (refer to the up coming Wednesday, no matter it is in this week or next week)
If you meant to say "this Wednesday" (the Wednesday this week, no matter it is yet to come or has passed) you have to say "本星期三" or simply "星期三"
"下一個星期三" or "下個星期三" (the next Wednesday) is considered ambiguous only in theory that '下一個' (the next one) and '即将到来的' (the up coming one) carry similar meaning. But it is never ambiguous in practice "下一個星期三" or "下個星期三" (the next Wednesday) is considered ambiguous only in theory that '下一個' (the next one) and '即将到来的' (the up coming one) carry similar meaning. But it is never ambiguous in practice
[下 (next) + day of the week] is in relative to [本 (this) + day of the week]
下 = next (week's)
本 = this (week's)