effect of overpressure on lowering viscosity of polysaccharide solution
The pressure is irrelevant, it is only applied so the solution can reach 110°C. Water boils at 100°C at ambient pressure.

Agarose is a polysugar, and at 110°C in water, I expect it breaks down into smaller molecules, disugars, monosugars etc.

Solutions of smaller molecules have lower viscosity. https://en.wikipedia.org/wiki/Mark%E2%80%93Houwink_equation

For typical polymers, the scaling exponent $a$ of Mark and Houwink is 0.5-0.8, but that's for infinitely low concentrations. It goes up drastically for higher concentrations, reaching a typical 3.4 in the melt.

(That's a bit sloppy, Mark-Houwink predicts the intrinsic viscosity, while in an entangled melt the actual viscosity scales with $M^{\approx3.4}$.)

So, if you have a saturated solution, and the average agarose polymer molecule breaks just twice, a factor ten in viscosity decrease is very reasonable.