I watched Rule #1 in a theatre, and it was in a Mandarin track. But that’s because in Singapore, the “speak Mandarin campaign” has eradicated all chinese dialects from the media. So we have it in Mandarin instead, and undoubtedly, these versions are dubbed.
DVDs here on the other hand, could be released in the Mandarin track and in some cases, comes with the Cantonese track as well. Don’t ask me why the double standards, but such is the situation here for as long as I have lived 
So yes, when I watched it, the Hong Kong actors were mouthing their words in Cantonese, while the Singaporeans could be doing so in Mandarin, and then having their voices dubbed for whichever market it will be screened at.
I believe the folks you mention do speak Mandarin to a certain degree, though they may not be proficient or hitting the correct diction each time. But I guess different markets call for different track requirements, and with Pan-Asian/Chinese collaborations, it’s becoming too common to be disturbed by it.
For example, in Benny Chan’s Connected which was screened here in Mandarin, Louis Koo was obviously speaking in Cantonese (and having his voice dubbed), but we get Mandarin as spoken by Taiwan’s Barbie Hsu, undubbed.