Taking some shots at the hot exhaust of an F-35, while a commendable achievement for a group such as Ansarallah, is not the same as shooting one down. At the end of the day, they shot down zero planes.
"Stealth" is a valuable technological concept that was first discovered by the Soviets in the 1960s, that an object's radar return is related to its shape and not only it's size, and thus can be minimised by changing it's shape.
Clearly it works, we've had many cases of stealth aircraft taking their adversaries by complete surprise, from the Gulf war to Yugoslavia to now. There are videos online now where you can hear Israeli aircraft in the background over Iranian skies.
Is it a Hollywood tier invisibility cloak? No, and it's possible to shoot down stealth aircraft. But it's a force multipler, which is why China is working very hard on their 6th generation stealth aircraft and their own analogue to the F-35, called the J-35.