This might sound like an insane question, but is the overall theme of 4e to say "yes" to everything, including things like the above bolded text?
Saying yes to things - and more specifically, not having enemies that turn off player's powers - is a specific design decision of 4E. It very much avoids Anti-magic fields, undead being immune to Sneak Attack, enemies that destroy every weapon that touches them, etc.
Now, note that this doesn't mean you
have to play it this way. Their advice is to come up with an explanation as to why Spinning Sweep works on an ooze ("you disrupt its consistency and splatter it about, inflicting the same penalties as the prone state until it takes a move action to fully re-form itself"). But if you put in more rigid interpretations, the game won't completely break down.
You just need to be careful - the reason they took such approach is that once you've made a ruling, you could completely undercut one player's character. If they are all about knocking things down, and walk into a dungeon filled with oozes, that's going to be a long and boring adventure for them.
Of course, some players enjoy that style of old-school play. My personal advice is that, if it is the sort of thing that appeals to you and your players, simply use moderation with it, I'm planning to include an enemy immune to Arcane powers in my Epic game, just for the novelty of it. They key is, I will probably have other enemies in that fight that the Sorcerer and Warlock can blast away at.
Feel free to have some encounters that might let particular characters shine - you just want to be careful that the other characters aren't completely deprived of options for long lengths of time. Give them other foes to combat, or simply other things to do - whether traps to disable, or the ability to assist the fight in other ways (cheering on the fighters in the Arena, etc.)
If used properly, it can make for a distinct and memorable encounter or environment. But if used poorly, it can be really frustrating for the characters who get 'turned off.' 4E shies away from such things - which I think is a good call, since it is very easy for less experienced GMs to just stumble across such monsters and throw them at the group without realizing that they are eliminating several characters from the combat. But as long as you approach such a design carefully, I think you can reintroduce those elements to the game without running into the usual pitfalls.