I still think it's definitely "Tler!"
As an English speaker, I naturally struggle to pronounce a word beginning with "TL" - not sure there are many, if any at all, of these in Western European languages. Saying "Tler" out loud to myself just now (don't worry, I'm alone in the house!), it comes out sounding more like "Kler", and if I say it really quickly, the end of the syllable comes out as a sort of strangled cluck, almost a "P".
So that's my continued belief. Sam Kelly's character, Geering, is intending to parrot his superior's "Heil Hitler", but in his haste to say it (possible to try not to draw attention to himself by speaking for too long), he ends up with one rushed syllable which is almost indistinguishable from "Klop".
Then again, maybe he'd had a vision of the future, and became a big fan of Liverpool FC's current manager.