Indeed, the current IG codex also makes mention of dividing Regiments specifically to minimize the damage any one Regiment can do if they go traitor. That said, there are exceptions to the rule and it's pretty loose to begin with. The Cadian 8th includes artillery, tank and mechanized units in addition to its' primary infantry focus, the Armaggeddon Steel Legion always includes at least one Tank company in their Mechanized regiments and the Valhallans are noted for being extremely artillery-happy, with their Infantry regiments equipped with heavy siege guns. That's not even getting into oddball cases like the Death Korps of Krieg, which institutionally include combined-arms forces while the Munitorum turns a blind eye because they're all fanatically loyal death-seeking clones or the Tallarns, who field Rough Rider companies in their Tank regiments (and every other regiment, too).
The moral of the story? The IG lets you get away with what you need to do to succeed in your specialized field, and exceptional cases get exceptions to the rules.