I don't believe anyone here will say what is, or is not safe for your loop, but I do believe all aquacomputer temperature sensors exposed to coolant are nickel plated brass. You should be able to do your research with that knowledge.

What is this all aluminum loop?

To get the coolant temp you don't need absolutely need to use one of the fitting type sensors. They are just convenient more than anything to my mind.

I have gotten better results by attaching the standard flat film sensors to metal parts of the loop like the end tank of a radiator.

The fitting type sensors just use the exact same film sensors mounted around or glued into a fairly standard fitting. So they are really sensing the fitting temp. Fittings have a relatively small surface area exposed to coolant vs their mass so they can trail the real coolant temp as it changes. They are pretty accurate of course with stable coolant temp. The end tank of a rad is thin with a large surface area vs mass exposed to coolant so it sticks better to real coolant temp. Even taping the sensor to an ordinary fitting works fine.

Copper is the way to go for loops though. It just seems to me that the cost isn't that much lower considering the performance gap. Still perfectly functional though of course if anyone prefers that route.

Or you could always submerge a thermal probe into the reservoir.