Learn French: Le renfort sera en retard. - The relief force must be late.

This is a perfect example of the futur de probabilité (epistemic future). In French, you can use the future tense to express a strong assumption or deduction about a present situation. While Le renfort sera en retard can mean "The backup will be late" (a future event), the context here implies a present deduction: "The backup must be late" (because they aren't here right now).

Why is renfort singular here? Don't we usually say "reinforcements" in the plural?

In French, the singular le renfort is often used collectively to mean "backup" or a "relief force" as a single entity. It is also very common to use the plural les renforts for "reinforcements." Both are correct, but the singular emphasizes the force as a whole unit.

Just to refresh, why do we use en retard here instead of tard?

Tard and en retard are not interchangeable. Tard refers to the time of day being late (like late in the evening). En retard means that a person, group, or thing is delayed or behind schedule. Since the relief force is delayed, you must use en retard.