Digital marketing is marketing through channels that can be measured and altered after launch. That, rather than the medium, is the real distinction: a billboard is bought and then endured, while a digital campaign is bought and then adjusted.
Screens and interactive video sit awkwardly across the usual definition. A signage network is a physical medium with digital economics: creative can be changed in an afternoon across every site, scheduled against weather or stock levels, and measured by dwell rather than impressions. Interactive video sits even further in, because every viewer generates a decision trail rather than a view count.
The caveat is that measurability makes short-term effects easy to see and long-term ones easy to ignore. Channels that convert people already looking for you will always outperform channels that make people look, so budgets drift toward the former until the pipeline dries up. Brand-building work is genuinely harder to attribute and genuinely still necessary.
The related trap is measuring what the tool reports rather than what the business needs. Impressions on a screen network are close to meaningless on their own; dwell time, interaction rate and downstream sales in the same store are the numbers that survive scrutiny.