Apple's stock loses over $160B as analysts forecast smartphone decline
neowin.net
neowin.net
In other words: pointless trading article is pointless.
So decline in this setting means a decline in growth -- that growth is slowing down. Whether that remains the longer term trend is still a hard call to make, and the article makes no comments about growth in other geographic markets.
It's definitely disingenuous of the article's author to make generic statements about all smartphones or statements that at first blush seem to be about the general worldwide demand for smartphone technology. But this article isn't really a robust analysis of anything; just clickbait to feed confirmation bias of market pessimists and spread more FUD to befuddled mainstream news patron types.
Apple's subtle shift towards leasing phones instead of carrier-subsidized purchases hints to me that they already know what's up in the marketplace.