Ha! Actually no of course I cannot - because if the tempo jumps is during a block then the increment will be wrong since it should take into account the new tempo (which the plugin will receive only on the next block). Hence the idea of using timeInSamples, which works in most places expect in Ableton.
The biggest irony, is that the reason I need to detect if the playback is continuous is precisely because of the fact I cannot get the tempo at an exact sample but only on the current block.
Maybe one day the VST3 interface will give a way to get a tempo map - gosh that would make my life so much easier… (it’s actually possible in an ARA plugin, I went down that way but not great or easy for the end-user, and not available in every DAW 
)
PS: just FYI I found this topic: CurrentPositionInfo.timeInSamples and tempo changes in ableton live - it seems this has been an Ableton bug for a looooong time.