Theyre out of plane shift (thats how tehy got here) so banishment works fine.
Firstly, because the Bard can clearly see the Slaadi and is not surprised - this may very well cause one to decide to lob a cloudkill instead of a fireball as a result. Cloudkill obscures the Slaad from the PC that can see them, granting them greater freedom of action, and regains the initiative (in a figurative sense) by sowing confusion.
Also, at least some party members each have potentially two saves to make against the CK due to surprise, and the Slaads initiative relative to thiers (on rounds 1 and 2). This means they may very well be forced to make two saves.
Plus - the bard is in leather armor - from the looks of him he's probably a rogue of some type and thus good with Dex saves, and likely has evasion.
Secondly, the DM is well within his rights to metagame. He's the one behind the screen. Its his job to deplete party resources, police the adventuring day, and maintain the overall challenge and balance of the adventure. He's within his rights to throw a 'random encounter' at the party (that isnt random) if theyre having too easy a time of it, just as he's within his rights to fudge a lucky hit on a PC that could result in an unexpected and unfair TPK.
Its part of running the game. Im aware you probably prefer your DMs to be less participatory and more passive (basically just referencing the rules, and staying out of the story) but I violently disagree with that assertion. The DM doesnt just reference rules, he also (with the trust and participation) regulates the pace of the game, the challenges faced and so forth. There are whole chapters devoted to this in the DMG. Its not your preferred DMing style I can tell, but its a perfectly valid way to DM the game (again, see the DMG).
Putting my DM hat on here, I might decide (as DM) that as the party are fairly resource heavy (having breezed through the earlier encounters) to hit them with a harder effect to increase the overall challenge of the rest of the adventure. I pause and think of an 'in game' reasoning for this strategy (forcing two saves against cloudkill, and circumventing this foe who can clearly see the Slaadi by shutting down the visiblity of the party makes tactical sense) and come up with 'The Slaadi see you charge forward holding a bright gem and reacting to their presence - you see them stop chanting the spell they were about to cast... however they instead cast a spell you recognise instantly - Cloudkill! Make a Con save please as noxious green vapors blocking out vision choke up the hallway'
And cloudkill would have been countered. What part of that is not being understood?