Political Cartoons

The books is actually quite explicit about what makes the Fremen such good fighters - their hard environment shapes them.

Putting aside all the issues with that premise and accepting it as an axiom of Herbert's world, you are quite right.

But precisely because it's such an inintuitive notion that a force of desert-dwellers shaped by scarcity, lacking both the equipment and the training for shield combat and the rapid troop transport (that he knows of) to stage a surprise attack could suddenly attack and overcome his well-fed, -trained, -equipped professional army, I'd absolutely give him a pass on not expecting that too. What makes the Fremen so dangerous in the desert is that shields are useless and even dangerous out there, while they know the dangers and can use the terrain to their advantage. But in the city of Arrakeen?

The abilities of the Sardaukar were largely attributed to the harsh environment and brutal discipline they were exposed to on the planet Salusa Secundus, the Corrino prison planet.

Again, the Sardaukar also have dedicated training and equipment, which would give them a decisive edge over the unshielded Fremen.

The battle of Arrakeen, of course, happened without shields and generally on favourable terms for the Fremen, orchestrated by a prescient proto-mentat and generally fucking up the balance fiercely.