That would be because we are no longer playing 5th edition, and the ruling that was made in 5th edition was not carried across to 6th. Which means it ultimately comes down to personal opinion as to whether they just forgot all about this issue when writing the 6th edition rules despite having previously FAQd it, or if it was a deliberate change for 6th.
... and my real thought is are people seriously going to be giant Douche Nozzles about it or just go with how the rule was meant to be used?
Given the ambiguiity I just mentioned, it's not at all unreasonable for someone to refuse to allow this. Again, we really have no way to know for sure just what
GW intended for 6th edition.
I mean, why else would it be stated the way it is AND with a special character (shrike) that's whole reason for being is to infiltrate with a unit that doesnt have it?
Shrike's rules were written for a previous edition. And as others have pointed out, the rule as written covers mixed units like Wolf Guard with Scouts. It doesn't specifically have to be intended for
ICs.