asabjorn's claim was that ag productivity was directly sabotaged.
That is a highly unlikely reading of a situation I see as:
1. Government which supported fertilizer imports through direct government spending and via reserve foreign currency holdings was unable to support either end of this policy: it had no capacity for funding imports and had rapidly diminishing foreign currency reserves.
2. Amongst other measures taken, fertiliser and pesticide imports were reduced. Because of financial necessity, not as some "degrowth is good" initiative.
3. Resulting ground truth was that ag productivity collapsed, as you note.
Again, the driver was not "let's de-grow the economy", but "we can't afford what we had". In the context of other comments I've made on this thread, what Sri Lanka are experiencing might better be interpreted as an unplanned collapse rather than a longer-term, planned, and engineered de-growth. Sri Lanka's fertiliser ban itself was a consequence, not a driver, of unplanned collapse.
The usual poster-child of the alternative viewpoint might be Cuba. That country also saw a dramatic reduction in foreign support when its patron state the Soviet Union itself collapsed. There was a dramatic and difficult period of adjustment, but the result has been a stable and sufficient existence, if not one at notional OECD / major industrialised nation levels of wealth and income.