I wonder what reasons the guide gave for not wearing jeans. I don't wear them when hiking for several reasons because they are usually tighter than many other pants and this can lead to chaffing on long walks, especially so if they get damp or wet. Then, as others have mentioned, if they get waterlogged they take seemingly forever to dry out. My usual remedy is to wear army BD cargo pants, they're roomy and comfortab…
second hand army gear was always my goto for hikes and traveling in general. no need to verify whether the clothing was suitable, because, while there may be even better gear out there i can trust that army gear is most certainly not unsuitable. it's cheap and also already worn in so you avoid issues that you sometimes get with brand new clothing
Incidentally, some years ago I bought between 40 and 50 pairs of army pants at a military/disposals auction, some were brand new the others in very good condition (they're of Vietnam vintage). At the time it wasn't my intention to buy them but they were bundled together on a pallet with what I actually wanted, namely 50V telephone exchange type power supplies—and I had to take the lot. Seems, as is so often the Army's wont, pants and power supplies begin with 'P' so 'logic?' dictated they should be auctioned together!
At the time I was a bit pissed off at being landed with dozens of pants that I didn't want to be seen dead in (having been drafted some years earlier and being forced to wear them). Anyway, my reticence eventually faded and I wore them for everything, fixing the car, hiking, etc., etc.
In the end it turned out to be a bargain purchase, a really good deal. I've now just about worn the lot out, perhaps there's a half dozen good pairs left. Now I wish I had more.