I wouldn't say LG will capitalize on this, other than them making a decision that might help their supply chain. Both chips are going to be ready for use... Samsung isn't some noob when it comes to fab... it's part of their core business.
That being said, if there was a significant issue with production capacity, a week of delay wouldn't cut it. If the week delay was caused because of production shortages, those shortages are most likely relatively minor and something that they might have already resolved. Could very well be that the fab house was just a week behind in being able to output enough for full production runs.
And this would sort of make sense when you look at it through the lens of LG using the 821.... phones aren't produced instantly. Decisions take time to implement. The choice to go with the 821 over the 835 would have had to have been made quite some time ago, and at that time, Samsung might have issued guidance that there may be an indeterminant production issue that has since been resolved. But once LG made the choice, they had to stick to it.
Not that it matters... the 821 is still an excellent chip.... we're long past the point where the 'new' chip drastically outperforms the old one. Functionally speaking, you probably won't be able to tell much of a difference between the 821 G6 and the 835 S8... and even then, a good portion of those differences might have little to do with the hardware and have everything to do with the software that drives it.