It's called thinking of MULTIPLE SCENARIOS and not just those favorable to your cause
It is very possible to give a def boost
Mega Kangaskhan commonly carries Power-Up Punch, which is essentially a 70 BP un-Tauntable Swords Dance, and it commonly beats a lot of bulky switch-ins simply by hitting them with Power-Up Punch on the switch and following up with the appropriate attack. Lugia, on the other hand, does not commonly carry Reflect. It's not that it's a bad option, but it's pretty rare because there are so many other good options that compete for a spot on any given Lugia set. Not only that, but unless Lugia is running a lot of Spe investment (which, again, is less common than those that run more bulk instead), it won't be able to get up the Reflect before Kangaskhan hits it with Return and KOs. Assuming Kangaskhan is boosted and Lugia (or any other defensive switch-in) isn't is not an unfair scenario that's just favorable to the ban side. It's an extremely likely scenario. Kangaskhan very commonly gets to +2 because spamming Power-Up Punch for the boost is very easy to do. Even when there is a Ghost-type in play, simply hitting it with Scrappy Power-Up Punch on the switch will let Kangaskhan mega evolve the next turn and hit it with a +1 Parental Bond Crunch (alternatively, it could just mega evolve immediately and use Crunch on the switch for the 2HKO). If Lugia switches into Kangaskhan, it is very likely that it will switch into Power-Up Punch and get KO'd by the follow-up attack, or it is very likely that Kangaskhan already got to +2 earlier in the match.
Saying, "Well, Kangaskhan isn't broken because it doesn't break walls that easily when it's unboosted," doesn't make much sense when a +2 Kangaskhan is such a common occurrence. If a Pokemon is to be considered a "counter" to Mega Kangaskhan or to be said to "wall" it, it needs to be able to take it on in its entirety, not just in those few circumstances where Mega Kangaskhan happens to not get the +2 boost.