I don't know. But I can offer up some speculation. To do that we have to put in a guess about what "common" shares of stock would be worth, and how many of those shares our employees could access.
We know that Jet has been a bit unconventional in their pay practices [1] with their transparent salary metric. But we don't know how their options differ. Sometimes options have a "change of control" clause. In those cases the vesting schedule for common shares is accelerated when the company is acquired.
TechCrunch says "$3B cash plus $300M in shares to the Founders and employees". That sounds like part of the acquisition offer is stock offers to employees to come to Walmart. The new stock options would have cliffs and restrictions as well but they are for a publicly traded stock (so you know that you will be able to trade them, vs a private company where you may never be able to trade them). If the Crunchbase article is correct and there are 1K - 5K employees, we'll assume the distribution follows the power law but even with that, smallest grants would be at least $100K if distributed over 5K points with a $300M total.
Second there is the "value" of common stock. If the company had raised a total of $800M, and already had a 2x liquidation preference with participation, that would have $1.4B to distribute across the final common pool. In common scenarios that would mean that people who had been employed for a year or more would be able to exercise n/48ths of their stock option (where n was months of service as long as n was 12 or higher). The Tech Crunch article also said that they had been shooting for a $3B valuation in the last funding round. Given that jump you might expect common shares from the resulting acquisition to have a value that was at least 2.1 times their strike price. That might not represent a lot of money though, a million shares with a strike price of 10 cents a share, now worth 21 cents a share is a $110K gain. That said it would depend on how many shares were outstanding. If they had held back 20% of the share pool for employee options, and distributed half of it, that would be 10% of the company in options, or potentially $300M in value. That arbitrarily lines up with stock number quoted in the press release. If that relationship was accurate, it would suggest that Walmart was converting the options straight to Walmart share options (another common practice) where the acquiring company keeps the previous vesting schedule.
Lots and lots of variables. On the plus side, if an employee did have a stock option and it was at least partially vested, that option was probably worth non-zero dollars so in the world of startups that counts as a win. How big a win will vary from person to person.
[1] http://www.forbes.com/sites/erikamorphy/2015/07/21/jet-com-i...
[2] https://www.crunchbase.com/organization/jet#/entity
[3] https://en.wikipedia.org/wiki/Power_law