Jonathan Chait at New York magazine makes the same point:

The exchanges in 2017 had stabilized financially, as insurers found a profitable price point. The Republican Party has, as a matter of theological principle, refused to accept the possibility that Obamacare might succeed at its stated ends. If Obamacare were truly collapsing, sabotage would not be necessary. It is the law's success, not its failure, that has made Trump so determined to wreck it. The White House has released a statement confirming its intention to end the payments, written in the pidgin English indicating the president's own authorial hand.

I'm pretty sure Trump not only doesn't understand how insurance works, he has even less understanding of what the cost-sharing reduction payments are (this is, after all, the man who thinks the gains in the stock market are erasing the deficit).

For those who don't know:

The ACA requires insurers to offer plans with reduced patient cost-sharing (e.g., deductibles and copays) to marketplace enrollees with incomes 100-250% of the poverty level. The reduced cost-sharing is only available in silver-level plans, and the premiums are the same as standard silver plans.

To compensate for the added cost to insurers of the reduced cost-sharing, the federal governments makes payments directly to insurance companies. The Congressional Budget Office (CBO) estimates the cost of these payments at $7 billion in fiscal year 2017, rising to $10 billion in 2018 and $16 billion by 2027.

Not to mention that ending the cost sharing subsidies will likely cause premiums to rise.  As premiums rise, the tax credits will have to rise, as well:

Any systematic increase in premiums for silver marketplace plans (including the benchmark plan) would increase the size of premium tax credits. The increased tax credits would completely cover the increased premium for subsidized enrollees covered through the benchmark plan and cushion the effect for enrollees signed up for more expensive silver plans. Enrollees who apply their tax credits to other tiers of plans (i.e., bronze, gold, and platinum) would also receive increased premium tax credits even though they do not qualify for reduced cost-sharing and the underlying premiums in their plans might not increase at all.

We estimate that the increased cost to the federal government of higher premium tax credits would actually be 23% more than the savings from eliminating cost-sharing reduction payments. For fiscal year 2018, that would result in a net increase in federal costs of $2.3 billion. Extrapolating to the 10-year budget window (2018-2027) using CBO's projection of CSR payments, the federal government would end up spending $31 billion more if the payments end.

Here is another article that explains more about cost-sharing subsidies and their effect on co-pays and deductibles.