Do you have a source? Revenue and profit are completely different things, and I am curious if Pocket was making any profit. Otherwise it's losing money.
I suspect it's about the longer term profits, like any other corporation purchase. Generally, you can't buy/afford it when it's already super successful, but you can when it's on the verge of being that (but needs your funding/image/resources).
Normally companies raise venture capital in the cases you describe.
Acquisitions happen when the stakeholders (founders, employees, and investors) don't think they will get the level of return on investment they will be satisfied with.
Even if they DO decide to sell, the case you describe (on the verge of being successful but requires money to get to that point) sell at a disclosed amount. Undisclosed amount mostly means the investors and founders are not proud of the result so they would rather just keep it private.