'Would' is certainly used to convey near-certainty for past events:
- "Somebody let off a stink bomb in the Library yesterday."
- "That would be Tom" / "That would have been Tom."
The Cambridge Dictionary sense you give does not explicitly prohibit present-timeframe events, but gives an example in the past:
used to refer to what is very likely:
- "The guy on the phone had a Southern accent."
- "That would be [/would have been] Tom." {synonymous, as FF explains in a comment}
But statements about possible present-timeframe events expressing strong likelihood are also commonly framed using 'would':
- "There's somebody coming up the drive."
- "That would/will be Tom."
An example from the internet (though admittedly there aren't too many) is:
- A ding ringed out from her phone. "That would be him now, please let me respond." [Wattpad; repunctuated]
Perhaps opting for 'would' adds a slight doubt about how certain the speaker is about the statement, as you say.
But as you suggest, there is alethic modality at play here, not any overt or deducible condition.
Alethic modality is modality that connotes the speaker's estimation of
the logical necessity or possibility of the proposition expressed by
his utterance [and broadened to an estimate of probability] [Glossary of Linguistic Terms].
'Would' is also used quirkily to someone hesitating to address someone directly:
- "I've been asked to hand this petition to the headmaster."
- "That would be me."