It's true that there's a lot we don't know. Executive employment contracts are bespoke and quite different to peon contracts. There will be various incentives that might be cash or incentivized stock options ("ISOs") as well as regular compensation that might be RSUs, non-qualified stock options ("NSOs") or both.
But we do know:
1. The acquisition is imminent but not closed;
2. Executive contracts often include bonuses on acquisition and/or accelerations for change of control;
3. There are numerous examples of companies cleaning house to avoid payouts prior to an acquisition closing;
4. Generally a company on the verge of a likely acquisition just keeps the lights on and doesn't make any big moves so things like hiring freezes make sense; and
5. The circumstancial evidence that both executives just happen to be shy of their 4 year anniversary (in their current roles).
It's not uncommon for companies to "clear house" right before close of acquisitions, especially on exec level. However I don't believe it's for the reasons you mentioned (saving money). Instead it's likely to better align with new ownership (whether perceived or actual alignment) and ensure the company is well positioned for the change.
An established/relatively healthy org like Twitter will likely not be penny pinching at the risk of more fallout/attrition. In fact, I'm sure the convo went something like "if you leave now, you will keep xyz/golden parachute".
This has at least been my experience based on limited experience of being part of a few acquisitions and working closely with execs.
But we do know:
1. The acquisition is imminent but not closed;
2. Executive contracts often include bonuses on acquisition and/or accelerations for change of control;
3. There are numerous examples of companies cleaning house to avoid payouts prior to an acquisition closing;
4. Generally a company on the verge of a likely acquisition just keeps the lights on and doesn't make any big moves so things like hiring freezes make sense; and
5. The circumstancial evidence that both executives just happen to be shy of their 4 year anniversary (in their current roles).
Conclusive? No. Kinda sus? Absolutely.