大般涅槃經義記 10.68

10.68

自下第三明無罣礙。「如是無礙名為虛空」明喻似法。「如來得已一切無礙」辨法同喻。亦以如來得菩提時證窮法實，見一切法平等無礙，故就論之。「故言佛性猶如虛空」結法同喻。

From here below is the third point: clarifying freedom from obstruction. "Such unobstructedness is called space" clarifies the metaphor resembling the phenomenon. "Having attained it, the Tathāgata is completely unobstructed in everything" distinguishes the phenomenon as identical with the metaphor. This is also because when the Tathāgata attained bodhi, he fully realized the true reality of dharmas and saw all dharmas as equal and unobstructed; therefore, it is discussed in these terms. "Therefore it is said that the buddha-nature is like space" concludes that the phenomenon is identical with the metaphor.