If there is only one true Sacred Timeline according to the Time Variance Authority (TVA), then how could Doctor Strange have seen 14 million of them in Infinity War? Or did he only see the true timeline and didn't reveal it to avoid causing Nexus Events?
The most likely possibility is that the other 14,000,604 outcomes Strange saw would have resulted in a nexus event, and would have been reset by the TVA had they occurred.
Free will - or the illusion of free will - has been a major recurring theme of Loki. Loki is adamant that he's had full control of his life thus far, but the TVA are adamant that everything he's done (and will do) has been according to the Time-Keepers' designs. We also learn that
Loki has actually strayed from the Sacred Timeline on numerous occasions, resulting in the many Loki Variants seen throughout the series, with the TVA aggressively "pruning the branches" to ensure Loki's life proceeds as it should do.
So there really were 14 million possible outcomes, but while Doctor Strange believed that he had to do everything in his power to ensure the one winning outcome occurred, in reality that was the only outcome the TVA were ever going to permit.