I've found myself wanting to reach for the SPI version of this primitive numerous times in the past (the previous thread has some great motivating examples) so I'm very excited to see this finally being pitched as an official API.
Regarding the name, my only concern is that using the term "synchronously" might make people think that the API blocks for the entire duration of task execution. Another option might be startImmediately but I'm not sure if that's as descriptive (though it's worth noting that the proposal draft does use the word "immediately" 6 times!)