The story was published by the Sunday Times.
The relevant quote given to the Sunday Times was:
Officials at the foundation were reportedly 'stunned' after a lawyer for the Sussexes informed them they would not be going ahead with the event last week, shortly before the Netflix deal was announced. The lawyer cited a 'conflict' of plans with another streaming service.
A source involved in the planning of the event told the publication: 'It's very bad form and everyone at Invictus is gutted.
'Harry said yes to doing this last year and everything was still moving forwards until a few weeks ago. 'This was going to be a big moment for Invictus where the pot is pretty empty, and it has left them in the lurch. Harry needs to pull his finger out to find another way to raise funds for them.'
So basically, the view of the Sunday Times article was that due to an exclusive access clause in the Netflix contract, the involvement in the Invictus fundraiser had to be ended due to it being streamed on a rival streaming service - Amazon.
The exclusive access part is entirely plausible, if not a given. No way would Netflix pay that much money for the rights to use Harry and Meghan's image and then let Amazon have them for free. Same way athletes sponsored by Nike 100% cannot be seen in Adidas or Under Amour clothes etc.
Of course, there may also be other factors in a 2021 fundraiser being cancelled.
Overall, this is big business and big bucks and there are realities to commercial impacts of contracts. That's often where royalty and commercial interests conflict. Netflix, a corporation, probably can control to an extent where Harry and Meghan appear.