Explanation of Quran 25:55 and 55:19-21

In the Quran Allah says he released the two water source freshwater and salt water. Put barrier between them so they don't transgress. > He released the two seas, meeting [side by side]; (19) Between them is a barrier [so] neither of them transgresses. (20) So which of the favors of your Lord would you deny? (55:19-21 ) Quran 25:55 and 55:19-21. Ibn KATHIR and bunch of other scholars explained the barrier is dry land that prevent the saltwater transgressing against freshwater. Meaning sea doesn't destroy the freshwater sources. There is something call saltwater intrusion, it's due to sea lvl rising or strom or land eroding due to high tide or storm etc. (I am only referring to natural cause not human induced like climate change cause it etc.). So how to understand the verse when it says they don't transgress but here we know naturally there are time when salt water intrusion happens, and it destroys the freshwater sources. Also I looked into the explanation of salt water and fresh water don't mix due to saline lvl. This modern argument don't fit cuz given enough time it does mix. I don't mean to cast any doubts, may Allah forgive me for if I said something wrong?