Extra Attack Interaction Clarification

So I know extra attacks from multiclassing normally don't stack, but what about extra attacks confered from class options? Specifically Thirsting Blade/Devouring Blade Invocation on an Eldritch Knight by taking the Eldritch Adept feat multiple times since those are not extra attacks gained through multiclassing.

Extra attack and Thirsting Blade would not stack, you would be able to use either option of your choice (functionally the same, but the invocation requires a pact weapon), so you would only get 2 attacks per Attack action. In this particular case, it would be pointless to take Thirsting Blade if you already have Extra Attack until you get to the point where you could also take Devouring Blade for a third attack (of which Thirsting Blade is a prerequisite).

Thirsting blade/devouring blade require warlock levels and are thus extra attacks gained through multiclassing.

Darn I was hoping that would be the exception but this clarification is appreciated. Getting a pact weapon for other boons though is still viable and thusly not off the table.

The relevant language in every iteration of Extra Attack or an analogous feature is "when you take the Attack Action, you can attack X times instead of once". It's like with Unarmored AC features- they don't add to an existing pool, they create an alternative option you can utilize.

You never gain the Extra Attack feature "from multiclassing" or "gained through multiclassing" per se.  You just always gain the listed class features whenever you level up in any class.  The Multiclassing section of the character creation chapter just provides some additional rules when certain features are gained from more than one class.  In the case of Extra Attack, it doesn't stack.

The multiclass rules actually explicitly list this exact interaction from the OP as an example of how the Extra Attack feature from more than one class doesn't stack:

Similarly, the Warlock’s Thirsting Blade invocation, which grants you the Extra Attack feature with your pact weapon, doesn’t give you additional attacks if you also have Extra Attack.

It was the same with 5e rules:

Extra Attack

If you gain the Extra Attack class feature from more than one class, the features don't add together. You can't make more than two attacks with this feature unless it says you do (as the fighter's version of Extra Attack does). Similarly, the warlock's eldritch invocation Thirsting Blade doesn't give you additional attacks if you also have Extra Attack.