Your concerns are valid:
The provided sketch would not pass any plumbing code that I am aware of. It indicates a vent from a separately vented fixture connecting to the drain portion of another fixture.
The upper end of vents need to connect to other vents or terminate to open air. They cannot connect to the drain portion of a drainage system.
Regarding other comments that the provided sketch illustrates a wet vent: It does not. Wet vented fixtures share the same drain pipe. There is no dry potion of vent between the fixtures and the wet vent potion is sized as such.
The below sketch shows the two common vent options for this scenario.
The left sketch illustrates two separately vented fixtures connecting to a single common vent. Note that the vent of the lower sink can offset horizontal once it is 6" above the flood level of its sink and it can only connect to the upper sink's vent 6" above that sinks flood level rim.
The right sketch illustrates a wet vent*.
*It should be noted that a wet vent as I've depicted could only be used provided that the sinks are not already park of a wet vent with the other fixtures in the same bathroom (this is typically the case).