I overpay a LOT for my AT&T plan. Somehow I have convinced myself that while they may be more targeted, they would likewise have the resources to address these attacks--something the low-cost providers may not.

But every time I see an ad for one of the alternatives, admittedly I think about whether this is a foolish rationalization...

I've been waiting to hear what they are going to do for us. So far, crickets.

..... this level of attack is carried out by state sponsored actors.

First, For all practical purposes, they have unlimited money, unlimited infrastructure and unlimited expertise. That means no carrier is really immune, they are all about equally vulnerable.

Second, For this level of attack, they are not interested in you. They are interested in high level state secrets and advanced technology development, especially with military applications. (I am assuming you are not a high level cabinet official, military commander or a scientist at DARPA. If you are, sorry for being presumptuous.)

For more normal levels of attack l would guess all of the carrier are competent, but I've never looked into it.

...because practically everyone's data on AT&T was hacked
just a couple of months ago...

https://www.cbsnews.com/news/at-t-data-breach-records-calls-texts-was-i-affected/

Or are we just supposed to take their word for it?

There was mention in the OP excerpt about it being "contained" with no further activity detected (at least for Verizon) -