That's ridiculous. There aren't very many of them and they're busy people and not exactly the most reactive or accessible bunch. There's a mountain of money waiting to be used to improve NEM. If an outsider can do it then it should be allocated. Ideas come from everywhere.
I have no clue or opinion about this project, but for instance the upcoming Trezor integration comes from an outside programmer. He did a deal and is being paid by developer funds as far as I know. It's a vast benefit to everyone that wouldn't otherwise have happened.
And this project isn't asking for all the funds upfront anyway.
There were few good points ...
Basically, I would not reject new devs, because they are coming "outside" of NEM.
But the another issues are
a) the possible benefits to NEM
b) the costs
To evaluate the possible benefits, there are needed good examples of the use cases, etc. 5 days is quite short time to study and understand them well enough.
To evaluate the costs there are 2 ways:
how much coding work (men months) the project requires or/and
how much NEM community values the possible benefits.