As long as you're not actually working for a different company during this time, it's all good. If company doesn't like it, move on.
And if that doesn't work out then move on.
It’s not difficult to pull that off at a big company.
If you prove it, they may just expect more from you in the same amount of time.
If you can’t do that, they were right to refuse the 32hr/wk proposal. If you can, just do it, sneak in some studying, and act like it took you 40.
For example, if you're doing daily standups, then one hour of an 8-hour workday is doable. But if you want to learn something that requires from time to time that you concentrate deeply for a longer period of time, say 4 hours, then that's half a workday, which may well show on daily standups.
If you have a manager who has a tendency to show up at your desk unannounced and they glimpse you watching a video, they probably won't say anything or ask what you were watching, but may walk away having mentally applied a discount factor to what they think your rate of productivity might be.
Also, while siphoning-off may work well in a good culture of consistent non-rushed problem solving, it may not work so well in a culture where it's constantly management by crisis. Like tomorrow may be the deadline for delivering something in the context of a crisis. The crisis may be entirely made up, but you would still end up looking really bad if you didn't deliver. Then you're probably not going to spend an hour learning that day if you know there's a risk you will have to stay at the office for an hour longer as a result.
Instead: (1) Here are your priorities. (2) Tell me if you run into any blockers. (3) Let the team periodically know what they need to about what you're working on.