From my interpretation of the conversation, you thought the question was: "Should we rewrite the checkout page in React, yes or no?" And responded with "Why do you want to rewrite it in React?" (paraphrasing)
Meanwhile the manager thought they were asking: "Please discuss the reasons why we should and should not rewrite the checkout page in React, including your recommendation on a course of action" And you responded with "Why do you want to rewrite it in React?"
Spelled out like that it's hopefully a little clearer that the "why React" was part of the question. They don't know if the page is performing "well" or whatever that means (even from an engineering standpoint it's hard to define, render time? bandwidth? server CPU?). They might know if users are complaining about checkout being slow, but even then users tend to leave rather than complain. If you were to tell them that the checkout page was performing "poorly" and React could fix it, that's the kind of information they were hoping to obtain from this conversation.
Similarly in my machine learning question, my CIO was actually asking me "how do you think machine learning can benefit this business?" And I responded with "how do you think machine learning can benefit the business?" He didn't know, he was asking me. He had just heard about the wonders of machine learning and was fishing for how it could benefit this business. Just like your manager had heard about React and was fishing for how it could benefit the business.
If your manager were here replying to comments I'd advise them to remember that engineers in general (spectrum or not), tend to be the literal sort, and if you aren't getting the answers your looking for it's often useful to examine the words you used and ask if you are actually asking the question your hoping to get answered or relying on subtext. But for the engineer's side of this conversation, it's important to ask if the question your answering is actually what they are asking, or if you failed to parse their intentions.