Except what we see in RO, isn't an AT-AT, it's an AT-ACT, a cargo transport. So there would be some similarities and differences coming from an AT-AT. An AT-CT wouldn't need the heavy armor an AT-AT has, since it's not built for combat but for cargo transporting.
In fact, Canon AT-ATs have withstood TIE Fighter blasters which are similar to X-wing blasters in terms of power output. So already the AT-ATs can withstand starfighter laser cannons.
You also have the Ghost firing upon AT-ATs in Rebels, yet that was ineffective too, the Ghost even used torpedoes on it and the best those 4 did was bring the AT-AT down briefly, yet it was still operational.