Yes, because the stream still has to be delivered from a specific set of IPs (some of those registered to YouTube) to a specific device (your phone, as identified by its MAC address or consistent IP address on the local network). It's a reasonable guess that YouTube uses a consistent set of source IPs for its thumbnails, so IT will be able to identify it.
Quality network hardware can count the packets (total bandwidth used over time) going to specific devices on the local network. The specific IPs of destinations on the Internet can also be recorded, so that IT can see "Bob visited Facebook at 10:32 and again at 2:17". Maybe they care, maybe they don't. Depends on the company.
Companies have several reasons to limit or ban Facebook, YouTube, or other "fun" things at work. Employees have sold their time and dedication. It's a distraction from the many useful things they could be doing for their company. It's a drain on network bandwidth.
The bandwidth is probably the thing that IT is concerned with. As long as you're using a company resource (its fast unlimited Internet), they are concerned with how it's being used. If you're watching full-screen vids all day, it's a serious drain on bandwidth. You're using thumbnails so it's a lesser drain. If you do it all day, they may ask you to limit it. Or, depending on your company, they might talk to your boss about how much time you're spending watching vids. Your boss will have to weigh your productivity versus time spent watching YouTube. Some companies are strict. Your company hasn't banned it outright, so it sounds like a middle ground.