Sigma bond behaving as stronger nucleophile despite presence of pi bond?

In my book, the following reaction is given 


[![enter image description here](https://i.stack.imgur.com/YMAbD.png)](https://i.stack.imgur.com/YMAbD.png)


The solution states that the sigma bond between hydrogen and carbon (3 degree) in isobutane attacks $\ce{H+}$ from $\ce{HF}$ to generate *tert*-butyl carbocation. This carbocation then adds on to ethene to generate another carbocation followed by some rearrengements until the given product is obtained.


My doubt is, since pi bonds are stronger nucleophiles (because of loosely held electrons) then why in this reaction the carbon-hydrogen sigma bond attacks $\ce{H+}$ instead of $\ce{HF}$ adding to the pi bond?




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**Source:** MS Chouhan; Advanced Problems In Organic Chemistry; 11th ed; Ref. Q.26 MS Chouhan - Alkenes


