وَلَقَدْ أَرْسَلْنَا نُوحًا إِلَىٰ قَوْمِهِۦ فَلَبِثَ فِيهِمْ أَلْفَ سَنَةٍ إِلَّا خَمْسِينَ عَامًا
The verse does not say "nine hundred and fifty years": alfa sanatin illā khamsīna ʿāmā — "a thousand years, fifty less".
This is a feature of style to which the lexicographers have drawn attention, and it is worth recording. I give this as my own reading: the large number is said first, the subtraction comes after. The figure that stays in the ear is therefore a thousand. That is, the expression is built so as to emphasise the length of the span.
And two different words are used: sana and ʿām. The lexicographers record a difference of tendency between the uses of these two words in the Qurʾān; I do not present it as a fixed rule.
فَأَخَذَهُمُ ٱلطُّوفَانُ وَهُمْ ظَٰلِمُونَ — and the outcome of so long a span is given in a single sentence.
I record this as an observation: the sūra recounts a prophet's nine hundred and fifty years of effort in a single verse — and immediately afterwards (15) speaks of those who were saved. Since the subject of the sūra is patience and trial, this span is set down as a measure.