Yes, this 25-year deal is a big deal, just recently signed and not getting much attention in the US media. Juan Cole has called it the most important deal involving China and the Middle East since the days of the Mongol Empire in the 1200s, when both what was then Persia and China were actually under the same ruler. This $400 billion deal was signed on the 50th anniversary of the opening of diplomatic relations between Iran (then under the rule of the Shah) and the Peoples' Republic of China (then under the rule of Mao Zedong). Cole identifies this deal as a "slap in the face" to the United States, or at least a clear sign of the limits of US power in the Middle East, with China stepping forward as a strong long haul rival.
I note only two points here. One is that on the one hand this is certainly a repudiation of US policy regarding Iran in recent years. It may be that its signing at this moment is a response to the failure so far by the new Biden administration to follow through on his campaign promise to rejoin the JCPOA nuclear deal with Iran. That really should not have been all that hard, but it increasingly seems that this simple matter has gotten bogged down in extraneous demands by neocons in the Biden administration, with both the US and Iran now having gotten themselves into a "face" conflict regarding "who will move first." I continue to hope that cooler heads are engaging in some unpublicized diplomacy, but all the noises so far have been that they are not. Both sides are posturing, but the US should have just moved. If this continues, it will be the most serious mistake of the Biden administration, and this move by Iran towards China seems to be part of this signaling.
On the other hand, I think that this deal, or something like it, was probably going to happen eventually anyway, even if Biden had done what he should and just rejoined the JCPOA and removed economic sanctions without any fuss. The signing might have happened later and the deal might have been smaller and more limited in certain ways, but Iran's position makes it a clear gainer from participating at least some extent in the Belt and Road Initiative (BRI) being carried out by China. Indeed, I think it is clear that Iran would be economically best off dealing with both the US and China and maintaining a balance between the two. As it is, this delay in getting back into the JCPOA by Biden may prove to have put Iran into a situation it prefers less, and certainly with the very stiff economic sanctions Trump put in place still in place, Iran needs some help now from any quarter, and China is willing to step in and has.