They didn't. Preservers used the stock AD&D wizard rules entirely, including casting times.
Later on, when the defiling-when-memorizing-instead-of-casting variant came out, defilers took half as much time to memorize spells. Which still left preservers using the stock AD&D rules for time taken.
Except for a few specific modifications to spells, there was, in fact, no mechanical difference between playing a wizard in the 2E Forgotten Realms, and playing a preserver in 2E Dark Sun. Everything worked exactly the same way. This makes it incredibly simple to do preservers in a 5E Dark Sun, because you just use the 5E PHB wizard, exactly as you would for 5E Forgotten Realms.
Yes, the Dark Sun setting material was full of talk about how preservers were careful, slow, etc. Yet, mechanically, they were no slower or more careful than Raistlin, Szass Tam, or Acererak.
Psionics were a major part of Dark Sun because everybody had a wild talent.
Psionicists were a distinct minority of PCs. Some tables might have actually had parties where a majority of players were playing psionicist class characters, but I seriously doubt those tables were common.
So, on the setting level, psion(icist)s are not actually necessary for Dark Sun. Wild talents, psi warriors, soulknives, and psionic-type feats are enough psionics to check the "psionics are everywhere" setting-feel issue.
They're not enough to fill the distinct "I am playing a member of the psion(icist) class" experience box, no. People who want to re-create that specific experience will feel it as a lack if there's no psion(icist) class, but given that most people who actually played 2E Dark Sun weren't having that experience when they played, and most of the potential audience for a 5e Dark Sun book (at least if it's going to do not-a-failure-for-5E sales figures) never played 2E Dark Sun anyway, it's not remotely essential.