Exactly this. Magnesium oxide is terrible at being absorbed, people need to take chelated Mg.

IIRC, Magnesium Oxide can be useful because it's poorly absorbed, making it a better osmotic laxative than chelated forms / free magnesium. I agree it shouldn't be advertised as benefiting anything other than that though.

Chelated magnesium is definitely better, I've also felt benefits from free magnesium (you can buy drink mixes w/ MgCO3 + Citric acid that fizz off CO2 when mixed).

Magnesium biglycinate and Magnesium l-threonate generally have the best effect with the least laxative side-effects (unless you want that); however, l-threonate is quite expensive because of patents, and l-threonate can have (potentially desirable, however potentially dependence building) effects of its own.

One form I haven't tried or researched yet is sucrosomial magnesium, which may have superior bio-availability to both biglycinate and l-threonate with relatively reasonable pricing characteristics.

I personally stopped taking magnesium a little while back, as it seems any bioavailable form pretty reliably causes insomnia for me. YMMV with magnesium, but you gotta make sure you buy it from a reliable source in a good form if you want to give it a proper chance.