In semi-strong form market efficiency, share prices reflect all publicly available information, and they react quickly and unbiasedly to new public announcements.

Here, the share price was fairly static before the profit warning, then fell 20% on the day of the announcement. That suggests:

The market did not know this bad news beforehand.

When the profit warning was announced (new public info), the price immediately adjusted.

That is classic semi-strong efficiency behaviour.

Weak form only says prices reflect past price data, not necessarily public news.

Strong form would imply even insider information is already reflected, so you might have seen movement before the announcement.

“Random walk” describes the pattern of price movements, not a level of efficiency in this context.