I mean they do say that for the first one it was after Zack's doa3 ending where he wins the jackpot with Niki, and in x2 it was after his doa4 ending where Niki and him found that Egyptian treasure.
I vaguely recall reading somewhere once before that they were considered non-canon, but I've no idea if that's true or not.
As shitty as the stories are, they still did try to fit them into the canon in some way though with the explanations that Jadeinchains mentioned, so it feels more accurate to think of them as canon, but who really knows.
Not to mention that two different people have been in charge. Maybe Itagaki saw them as canon, but the current TN doesn't, or vice versa. So again, who knows?
Either way, they do nothing for DOA's awful story anyway and canon or not, they can be lifted right out without making any difference.
The only thing we really gathered from them is that Hitomi is dumb as shit for falling for the same trick twice.
There is some points that actively counter its existance as canon, like doa x2 being after doa 4, breaks the "we haven't seen kasumi since the incident at doatech" or how "christie, you were the one that killed my mother" and in doa x 2 "christie, let's hang out and do friend things"
Or how between doa 4 and 5, zack has been an employee to Helena ever since the destruction of the tri building, and he has his own island on doa x3 and in doa x2. Also characters acting totally out of their usual personalities in general, but like i said, TN can just slap that in for some reason. EVERYONE LIKES TO POLEDANCE