There was a lot of hedging and backtracking in that video. The narrator also said that if the merger is not approved and there weren't any other bidders the stock could drop back to where it was before the tender offer. OTOH, he also said it could go to $13. So either way, he will claim to be correct.
No question the price would drop if tender offer fails. the only other offer astx received was in the $6 to $7 range, and even that was withdrawn after due diligence.
If the only company to make a bid refuses to go above $8.50/share that pretty much sets a near-term top.
Unlikely to get to that point, however. the buyer can extend the tender and announce they will not increase their offer. If that happens all the speculators who bought in the last week would sell and the price would drop below the offer price, making tendering an attractive option again.
Note that sarissa in their open letter said they were not tendering in order to give more time to seek new suitors. If they can't find one, you can be sure they will not hold on to their shares and suffer a fast loss
Actually even if the merger is not approved it is less likely to drop to a price where it was before the tender. Remember that before the tender there were many holders of ASTX that may have bought at a sigficantly lower price. Since the tender over 70% of the shares traded above tender price, therefore those who purchased at the higher price are less likely to be interested in selling at a significantly lower price unless there is bad news regarding one of the key clinical trails.