It's clickbait. Read the article in full and you see the following quote from the FA official who is their source for the story:
This is about prudent planning. Nothing more. Failure to be able to fulfil home games would lead to a huge fine and possibly a points deduction. So Spurs have paid a small sum to purchase an option to play at Wembley in the event of an unforeseen, last minute delay - e.g. tower crane collapse; industrial action; failure to gain health and safety certificates etc.
As to how ready or not the stadium looks to be in various updates, you'll find that most building projects with tight schedules look to be far from ready right up to deadline. There's no reason to believe that this project will be any different. As was always expected, this stadium will not be finished by mid September (and won't be fully finished and snagged for months to come). But it will be ready.
The interior bowl is complete, save for the installation of the remaining 20% or so of seats. The sliding pitch is complete, awaiting only the laying down of the turf. The roof is all but complete. The only respect in which the exterior of the stadium itself requires a considerable amount of work is the installation of the outer cladding. And since that is non-essential with regard to gaining an interim safety certificate, it won't affect the opening date for the stadium.
Of course, there is also work to do internally and on the public areas around the stadium. But both are well advanced and there is no suggestion that either are likely to delay the stadium's opening. As before, the critical distinction is between "finished" and "ready". The latter is all that is required to gain the necessary certificates.