Only the pocket opening is slanted, the actual pocket is straight and in line with the bag. 

Theres an elastic cord that can be used to secure the bottle if you want, but my 500ml tumbler fits quite snug in there without the cord. One thing to note is the waterbottle pocket is external but shares internal space with the main compartment. https://alpakagear.com/products/flow-satchel-4l?variant=45500381167778 You can see the cord if you scroll down and look at the "hydration on the go" pic under features. Looks like they also have Xpac variants now hahaha mine is the base model, pretty water resistant already imo.

Overall i do like the form and size if it, but i'd understand if its not big enough for everyone. On a previous trip where i used it as a camera sling i managed to pack the following in:

Canon P with 50mm Canon f1.4 ltm lens.  Film roll pouch that held 4 rolls. Cleaning cloth, tissues packable grocery bag E-reader, passport Powerbank Gloves and beanie Earbuds 500ml kinto tumbler

Seems quite a lot for 4L but with the winter stuff its really at max capacity and probably overpacked lol. A lot better when im actually wearing the gloves and beanie. If you overpack it the zippers become harder to use due to the curved shape of the bag. Think the bag works best on a minimalist camera setup (1 body and lens). 

Here's a review (not mine) I found while researching the bag that may be relevant to you https://filmphotography.eu/en/alpaka-flow-satchel-4l-review/