If you are aware that "Maxatar's conjecture is that 1 + 1 = 5", then it's correct to say that you have knowledge about "Maxatar's conjecture", regardless of whether the conjecture is actually true or false. Your knowledge is that there is some conjecture that 1 + 1 = 5, not that it's actually true.

In that sense, it's also correct to say that physicists have knowledge of relativity and quantum mechanics. I don't think any physicist including Einstein himself thinks that either theory is actually true, but they do have knowledge of both theories in much the same way that one has knowledge of "Maxatar's conjecture" and in much the way that you have knowledge of what the flat Earth proposition is, despite them being false.'

It seems fairly radical to believe that instrumentalism is indefensible, or at least it's not clear what's indefensible about it. Were NASA physicists indefensible to use Newtonian mechanics to send a person to the moon because Newtonian mechanics are "wrong"?

What exactly is indefensible? The observation that working physicists don't really care about whether a physical theory is "real" versus trying to come up with formal descriptions of observed phenomenon to make future predictions, regardless of whether those formal descriptions are "real"?

If someone choses to engage in science by coming up with descriptions and models that are effective at communicating with other people observations, experimental results and whose results go on to allow for engineering advances in technology, are they doing something indefensible?