Well, you probably know more about tanks than I do. But sorry, this article isn't about making tanks completely immune. It is about improving protection. To quote the article:

ERA blocks contain layers of explosives that explode outward when struck, potentially deflecting the incoming blast. Reactive armor doesn’t work against inert penetrating rounds, but against high-explosive rounds it can roughly double a tank’s protection.

With a tight layer of ERA, a Leopard 1A5 should go from having around 70 millimeters of steel protection to the equivalent of 140 millimeters or so. That’s still less protection than a Russian T-72 has, but it’s enough at least to give Ukrainian crews more confidence as they roll onto battlefields teeming with explosive-laden drones and anti-tank missiles.

You write:

Use them in a supportive role as a precise gun on range (and as it's rifled it is actually more capable in regards to long range precision than modern smooth-bore cannons).

Indeed, the Ukrainians use them as accurate long-range guns. But as for the "supporting role", what other tanks should these Leopards support? Perhaps Ukraine will miraculously receive or make hundreds of modern battle tanks in the future. But for now, the Ukrainians must do the best they can with what they have, upgrading old vehicles where possible.