It may result in an increase to CR
The DMG leaves a lot of ambiguity in the section on creating a monster. I suppose we ought to break this into two sections:
You should consider Damage Absorption to imply immunity for the purposes of calculating CR
Damage Absorption may not explicitly grant creatures immunity, but it replicates the effect (in fact, it is strictly more powerful). CR calculations are already murky, but we should at least be able to assume that the effect of an ability matters more than its name. Therefore, since it gives all the benefits of immunity, and has no added drawbacks, you should count it as at least equivalent to immunity for calculating CR.
The DMG likely assumes that a creature with Damage Absorption against a certain damage type already has immunity versus that damage type and therefore does not give it a CR increase. Additionally...
But it does not necessarily change the CR
Remember that the DMG section on calculating the effect of resistance/immunity on CR (p.277) says that
If a monster has resistance or immunity to several damage types- especially
bludgeoning, piercing, and slashing damage from nonmagical weapons-and not all the characters in the party possess the means to counteract that resistance or immunity, you need to take these defenses into account when comparing your monster's hit points to its expected challenge rating.
So the specific immunity matters in determining whether a CR increase is justified. For example, giving a monster immunity to acid damage is unlikely to justify an increase to CR, as acid damage is uncommon and the party is extremely likely to have alternate damage types. Therefore, acid immunity (and, in the same vein, acidic Damage Absorption) would not justify an increase to CR.
Since adding an immunity does not necessarily result in an increase to CR, it makes sense that they would not put a CR increase as a direct result of Damage Absorption; rather, this would be something the DM has to consider, based on the guidance given in the quoted section.