This isn't about Spencer's credibility in general, as if the subject doesn't matter.
If you're asking me if he can be trusted about this particular comment, I have more reason to believe it than distrust it.

That "blatant lie" you're refering to is standard industry PR. A few months before Majora's Mask 3D was released, Miyamoto said they had "no plans" for a 3DS remake at the time. They're not supposed to reveal game announcements or cancelations in random interviews ahead of time. They are instructed to give no new or definitive info in those situations. Some people call them liars. I just call it standard PR.
Those kind of comments can never be trusted. Doesn't matter who it is. Which is why I said this isn't a Spencer thing.

What he said about increased funding is different however. He said it spontaneously (wasn't specifically asked about it) and it does reveal previously unknown info. So this doesn't fall under the same category as replying to questions about game development.

How this is more of a PR statement?
The Scalebound comment is the standard reply to the question. The later isn't. That would be something like "We're still very dedicated to our first party output, and will work even harder to ensure that our players are satisfied." That would be ambigous.
But he went beyond the scope of the question and specifically added that they've increased funding as well. Like you said, there's no certain way to quantify that, but Microsoft is naturally aware of that as well. But in saying that, they willingly raised expectations on themselves. Whether it will show in the number of first party games they have for next year or in some other way, people will still look for it now. They certainly can't release less first party games next year after saying that, so that comment painted them into a corner in some regards. Which is why I don't think he would be that specific if there were no actual plans for it at the time.

This is different from the Scalebounds reply he gave, because he had to say that. If he had said anything else, headlines would be made speculating the cancelation of the game. And the Xbox division had decided on a certain date before they wanted that to circulate in the media.

The reason he can say something fales about game development is the same reason any other developer or representative can do the same. Because they've done this for decades. And those who understand how PR works will understand that they were not allowed to indicate anyt new developments at the time.
By your definition, we can't trust anything anyone in the industry says, because if they were asked about a game they're not supposed to reveal new developments about, they would have to lie. But I don't think that's entierly true. I look at the context of what is said, and rather than dismiss everything as a lie, I can consider some things more likely than others, and some things less likely than others.
Calling them liars is redundant.