Yeah, that Trumpian factoid has long been debunked. Formally true, but 2% is also an aspirational goal, to be met a decade from now. That parallels, at worst, Trump making it harder to meet the Paris pact's requirements a decade from now.
Those agree with him. The 2% is an aspirational goal. Here is an actual quote from the Economist article you linked to:
"At a summit in 2014, NATO reiterated its commitment to the 2% target. Members that fell short at the time promised to meet their obligations by 2024."
Here is the actual text of the relevant portion of the statement:
"Allies currently meeting the NATO guideline to spend a minimum of 2% of their Gross Domestic Product (GDP) on defence will aim to continue to do so. Likewise, Allies spending more than 20% of their defence budgets on major equipment, including related Research & Development, will continue to do so.
Allies whose current proportion of GDP spent on defence is below this level will:
halt any decline in defence expenditure;
aim to increase defence expenditure in real terms as GDP grows;
aim to move towards the 2% guideline within a decade with a view to meeting their NATO Capability Targets and filling NATO's capability shortfalls."
Whoever told you that NATO countries were failing to meet their commitments was lying to you so that you would support them politically. Consider not trusting them.
"At a summit in 2014, NATO reiterated its commitment to the 2% target. Members that fell short at the time promised to meet their obligations by 2024."
But, again, even in the worst light possible, a country not spending enough on military spending to meet some stated target doesn't compare to tossing out treaties based on a whim.
From the perspective of the US Constitution, the Paris Accords are most certainly not a treaty. Treaties require a 2/3 vote of approval in the Senate.
The tradition of significant international agreements made deliberately short of "treaties" goes back to FDR/Churchill and the Atlantic Charter, if not further.
The proof is in the pudding, here. This is a giant bat signal that the USA is unable to uphold its international obligations. Preen about the Constitution all you want, but that doesn't change the fact that people are going to take the USA's public commitments with a giant grain of salt now, and this will shift smaller countries' orientation to relatively trustworthy large countries, many of which are actively hostile to American interests.
The main confusion seems to stem from the fact that the 2% goal (agreed upon in 2014, to be met by 2024) is not "money for NATO", but how much eah member state shall spend on their own armies.
"Allies whose current proportion of GDP spent on defence is below [two percent] will... aim to move towards the 2% guideline within a decade...." [emphasis mine]
But of course you know all that already.