and yet that Wikipedia article notes that he "assigned a military commission to assess the feasibility" of the proposal rather than rejecting it out of hand. That suggests to me that a few relatively minor Mexican butterflies could swing things in the other direction.
A few things to remember here:
(1) Carranza assumed that the telegram would stay secret. Thus, there was no
need to reject it immediately out of hand. He could ask his military experts about its feasibility--and IMO unless they were all being paid off by the Germans, they could return only one answer.
(2) Carranza was a cautious man in dealings with the US--note how in the Plan de San Diego, he was careful to maintain "plausible deniability" if indeed he was personally involved at all.
https://www.alternatehistory.com/forum/threads/carranza-and-the-plan-de-san-diego.420152/ Note also that the purpose of his support of the uprisings (if he did support them) was not to reconquer Texas--it was to get arms and recognition from the US.
(3) Paradoxically, his failure to reject Zimmerman's offer out of hand (as long as it remained secret) was part of that same caution. Carranza feared that the US still might send troops back into Mexico and wanted to know if Germany
in that event really would help Mexico create trouble in the southwest US. [1] But as long as the US did not intervene against him, the chances that he would really go along with Zimmerman were IMO negligible. Once the telegram became public he had no choice but to disavow it. (BTW, the Germans renewed the offer after the US entered the war; predictably, the Mexican government replied that it would maintain strict neutrality.)
You write "Yeah, of course Mexico gets it ass kicked. That's not the problem for the U.S..." But it
is the problem for Mexico--which is why Mexico is extremely unlikely to go to war unless the US stupidly provokes it.
[1] "Carranza still feared the return of American troops and hoped to be able to make trouble in the southwestern United States
if renewed American intervention did occur." [my emphasis] Arthur S. Link,
Wilson: Campaigns for Progressivism and Peace, 1916-1917, p. 433.