"ُThe plotted their scheme...though their scheme is not (strong) enough to remove the mountains" OR "even if their scheme were enough to remove the mountains"
In this reading, the particle in is for negation, and the 'mountains' may be symbolic of the Prophet's (ﷺ) mission which will withstand their plots. Alternatively, in is conditional, so this is similar to the other reading.
لَتَزُولُ
la-tazūlu
"...and God has hold of their scheme, even though their scheme is certainly enough to remove the mountains"
In this reading, the particle in is lightened from emphatic inna, and this implies that the Prophet (ﷺ) is stronger than the mountains.
Maelezo:
These readings describe the unbelievers' scheme in contrasting ways; combined, they show that the scheme is very severe (see 71:22, 19:90) but not powerful enough to stop the truth from winning out [al-Alusi].
He has revealed to you ˹O Prophet˺ the Book in truth, confirming what came before it, as He revealed the Torah and the Gospel
Maelezo:
These readings describe the unbelievers' scheme in contrasting ways; combined, they show that the scheme is very severe (see 71:22, 19:90) but not powerful enough to stop the truth from winning out [al-Alusi].