How do I address 'As I said...' comments in team chat messages?

From all your points, I am missing the one that would lead to a solution of your problem:

Tell the other developer that you're sorry, you were told the endpoint was ready. Then go to whoever told you this endpoint is ready for development and ask them what you should do, now that you have been told it isn't.

Then act accordingly.

To me it seems you get those answers because someone told people they could start working against that endpoint. My best guess a project manager who didn't listen to the other team telling them it's not ready yet. Instead of wondering what the best response to a teams chat is to save your face or theirs, go to the source of the problem and fix it, so this does not happen any more.

Get Clarification

In these types of situations you lack far too much information to draw a conclusion to what is going on. It is best to ask the coworker in person or a direct message if in person is not available question(s) to determine what is going on. For example:

I have noticed you frequently mention things in chat about 'as previously discussed' and 'as we said', but I have looked around and I cannot find these previous conversations. Is this stuff documented in a different system, did these discussions only involve team leadership, or am I missing something obvious?

This shows that you have been trying to not put additional pressure on their team, and their answer will help you determine what is actually going on with their responses. For example it could turn out that every single person from your team is independently asking the same question via direct messages and not sharing this information amongst your team. If that is the case then your team needs to have a sit down and figure out who is going to be the liaison with the other team and be up to speed on what the other team is doing (or not doing).

A fair way to respond to what you're seeing is to ask the responder to copy and paste the "said" discussion into an email and to send it to you for clarification. As a matter of fact, don't ask for it in the discussion. Request it by email, so that you begin to have a paper trail:

"Hi XXX. On the morning of AA/BB/CCCC I asked about ???? via chat, and your response was 'as we said, _'. I wasn't a part of that discussion. Are you able to elaborate on that discussion? If it was in chat, I'd be okay if you did a copy-and-paste of the conversation. Thanks."

This approach is non threatening, but also something to turn over to your boss if you're not getting the information you need.

I am going to tangentially answer this - I asked a question in the comments and the subsequent discussion has very much confirmed a number of things (in my mind at least)

So - to start with, I am going to talk about the phrase "As we said" - this can also include "As per my email" and various other iterations.

Is it exasperation?

No - Exasperated is far too harsh - but it certainly is short hand for:

"You have already been provided with the information, you have not been furnished with an update that counteracts that - so stop asking questions to which you already have the answer to"

Now - you asked how to handle it - here is the answer...

You make sure you check the information that has already been provided before asking questions

I don't know your position in the organization, but if someone said that to me I'd message them individually and ask them who pissed in their Cheerios, that I'm not a kid to be F'ed with and feel free to publish expected results because I don't know who they are saying these things to. And the next time they reply "as we said" or some similar passive-aggressive bullshit on a public chat I would flame them with the heat of a thousand suns.

I'd further tell my boss that I'm about tired of their sorry attitude, and if they want me to work with this group they need to make some changes or put me in charge of them so I can.

It takes exactly zero effort to be nice. My response is in kind when someone isn't, but only privately until someone forces my hand.