No, IC can only use the attack listed in their description.
First, as you correctly state, Scramble IC doesn't list any effect for its attack when the host doesn't have three marks. And because IC isn't a "full" persona like a decker, and can't take other Matrix actions (not counting Patrol IC), all it can do is the attack listed in its description. This is heavily implied, if not explicit, from the way the description of IC on p. 247 only ever discusses the IC using its specific attack:
IC rolls the Host rating x 2 for any attacks, limited by the Host's Attack rating. The attack is a Complex Action, and the Dice Pool to resist the attack is listed with each type of IC. As with all Attack actions, a failed attack causes damage to the IC. IC is always considered to be legal, so its attacks never give it an Overwatch Score.
The use of the term "immediately" may be to clarify that it works differently from the Reboot Device action (p. 242), which reboots your device at the end of the following combat turn (ignoring the fact that technically it targets a device, which is hidden behind your persona, which means you can never reboot your own deck or commlink). But I don't think it implies that Scramble IC actually has any way to take the Reboot Device action or otherwise reboot you on a delay.
This is consistent with other IC that mimic Matrix actions.
IC that has the same effect as a Matrix Action usually requires the same number of marks, or gains the same increased damage from marks.
- Scramble / Reboot Device: 3 marks.
- Track / Trace Icon : 2 marks
- Crash / Crash Program: 1 mark
- Black/Blaster/Killer/Sparky / Data Spike: +2 DV/mark
From a balance perspective, some of these are nasty effects. Scramble in particular is a good way to ruin a hacker's host dive on a single failed defense test. So it makes sense to have an escalation period where the hacker has a chance to deal with the problem before getting dumpshocked. If the host wants the most powerful effects, they should combine these IC with Probe.