Hi Andrew and welcome back to the Disney Parks Moms Panel!
I enjoy staying in one of the Epcot Area Resorts and have walked between Disney's Hollywood Studios and Epcot several times. I have also taken
Water Transportation between parks when I have needed a break from walking.
I do think 35 minutes may be a close but hopeful estimate of how long it takes to walk from Disney's Hollywood Studios to Biergarten Restaurant in Epcot. The walk from gate to gate will take you around 25 minutes if you walk at an average pace. Once you reach the International Gateway at Epcot, you will have to go through security and the park entrance. Since the International Gateway is a smaller entrance, there are fewer lanes available for security checks and fewer entry points. It could take a minute or two to enter the park or it could take fifteen minutes (or more) and there is no way to predict it. Let's say it takes you 5 minutes to enter the park, then you will still have about a 10-minute walk to Biergarten. Those times total up to 40 minutes. I would give myself at least 45 minutes to have a tiny bit of wiggle room.
I think My Disney Experience app probably suggests an hour because they believe most guests will choose to take a boat. In my experience, I have not passed many people while walking between the parks. The boat takes longer than walking because after departing Disney's Hollywood Studios the boat stops at the Walt Disney World Swan and Walt Disney World Dolphin resorts, Disney's BoardWalk Resort, and Disney's Yacht and Beach Club Resorts before continuing on to Epcot. When you add in the time required for those stops, it quickly adds up to an hour.
Andrew, thank you for visiting us again. I hope you'll let us know if you have any more questions!
Wishing you a magical vacation,
Amy H.