Then by the same logic a Boost added by Engine Upgrade would be an action from an upgrade card and thus usable by EI and not PtL.
No, EU specifically adds the boost icon to the action bar. Had it said: action: perform a free boost action, then you'd be right. But it doesn't, so you're not.
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They probably worded it exactly like they did to put it in range of PtL.
(let's ignore the whole 'you can't equip EU and EI on the same ship unless you're into wasting points on an Interceptor." conundrum
)