I think, to get technical, things fall into the following categories
A) you use only licensed Traveller rules books with no changes and it is 100% Traveller. Perhaps with a possible list of optional elements that are or are not utilized like iron man or point buy chargen, knockout blow, alternate ship drives, with supplements "x-y", without books "a and c" and so on...
B) you use only licensed Traveller rules books with personal changes and it is Traveller with house rules. Is homebrewed Traveller the right term?
C) you use licensed Traveller rules books and rules from other game systems too and it is a Traveller hybrid
D) a combination of B and C. A homebrewed Traveller hybrid.
E) the only way a game could 100% be considered not to be traveller is if no part of any licensed Traveller rule book is used
For everything in between A and E, everybody has their own opinions as to what does or doesn't feel like Traveller.
It's just a matter of opinion, like what someones favorite color is. Just because you don't agree with someone and have a different opinion doesn't mean anyone is wrong.
EDIT: A) could be Traveller 5, MGT, Hero Traveller and so on. It also might include those Traveller games which use parts of multiple Traveller versions. But on this publishers own website forums I assume if no qualifiers are mentioned we are discussing a game that is 100% Mongoose Traveller.