To be clear, this was/is a statement about the worst case, not every problem. So it may be clearer to rephrase your sentence as:

For nearly 50 years theorists believed that there exist problems taking t steps that also need roughly t/log(t) bits of memory.

(The Quanta magazine article https://www.quantamagazine.org/for-algorithms-a-little-memor... gives some context: for a long time, the best result was t/log(t) as proved in "On Time Versus Space" https://dl.acm.org/doi/10.1145/322003.322015 by Hopcroft, Paul and Valiant in 1975.)