In the specific case you linked, there was an answer posted 3 days ago, which has been deleted. You can see this on the question timeline or simply by clicking on the last activity. However, it is shown only if you can see deleted posts (which is privilege of 10k+ users and moderators).
See also: Last activity on question shows modified by user who did not ask or answer
EDIT: One think which also users bellow 10k can see is that when you click on last activity (on the timestamp shown in the list of questions sorted by acitivity in the question next to the word active), then the url changes to an url of an answer.
For example, this question has at the moment deleted answer as the last activity. If you click on the timestamp then the url changes from https://math.stackexchange.com/questions/84587/given-g-group-and-h-le-g-such-that-gh-2-how-does-x2-in-h-for-ever to https://math.stackexchange.com/questions/84587/given-g-group-and-h-le-g-such-that-gh-2-how-does-x2-in-h-for-ever/84599#84599. (Notice the addition of /84599#84599 at the end.)
This no longer works for the question you linked in your post. (At the moment, the last activity there is retagging the question.) However, you may notice that the link in your post is a link to an answer. You have provided link in the form https://math.stackexchange.com/questions/1672134/uniform-continuity-uniform-convergence-and-translation/1888883#1888883 rather than https://math.stackexchange.com/questions/1672134/uniform-continuity-uniform-convergence-and-translation. So my guess is that you obtained the link by clicking on last activity (timestamp) at the time of posting your question.
