Now with that out of the way let’s discuss why brewers do not to spend any time at all worrying about the growth of Clostridium botulinum in the malt extract. Malt extract, whether liquid or dry, is concentrated by removing water. One key attribute of food products used to gauge their susceptibility to spoilage is a property known as water activity or AW. Pure water has a water activity of 1.0 and as solids content increases the AW decreases. The definition of AW is not important here, but relates to equilibrium relative humidity. If you want to read more there is a bunch of information about water activity online and in food science books.
At any rate, Clostridium botulinum is not a problem in foods with an AW less than 0.93 because it doesn’t grow. The water activity of liquid malt extract (LME) is somewhere around 0.60 depending on its concentration. Honey has an AW between 0.55 and 0.60, so it stands to reason that liquid malt extract with a similar concentration is going to be in the same range. Dried malt extract has an AW of about 0.20 making it very shelf stable from a microbiological view. You are correct that liquid malt extract is not pressure canned because there is no safety concern requiring it to be.