I'm not going to start quoting words from insurance policies. But for an umbrella to kick in, your primary Homeowners has to pay first. That' the exclusion you need to look for. Look under Motor Vehicles, its there. If its excluded under your Homeowners....your Umbrella isn't going to kick in anyway, they don't need to exclude it since your primary policy didn't cover it.
When I stated earler that you need to tell your insurer about anything with a motor I should have stated anything with a motor that moves you. There are exceptions to the exclusion for motor vehicles that service the premises and those to assist the handi-capped.
I can also tell everyone this, even attorneys on this forum, don't try to outthink your insurance company. They write the rules of the contract and they are all court tested. Just ask them if you're in doubt and do what they tell you. If they tell you that you aren't covered, believe them before something happens. If you have something that isn't covered don't wait for something to happen to tell them that you had an e bike, golf cart, atv....if it isn't covered they aren't going to let you buy coverage after something happens.
If you don't like what they tell you go to another insurance company. They advertise every 5 minutes on TV.