Yea, 5G does support carrier aggregation with LTE. It looks like the big difference is in LAA (License Assisted Access). LAA allows the use of unlicensed spectrum which can be added to the licensed spectrum that carriers use. 5G NR Release 15 (which is what networks are almost assuredly running since Release 16 just came out in July) doesn't have the ability to use unlicensed spectrum. One of the images in the PCMag article (linked below) shows the different combinations. You can see how LTE with LAA just has a lot of spectrum dedicated to it. 5G ends up with almost the same amount of spectrum as LTE for non-LAA LTE.
And it's important to note that right now, 5G isn't that much more efficient than LTE. Remember, when LTE came out, it was advertised as 5-8Mbps which we'd consider slow today. 5G NR will get better as it evolves. However, right now it looks like it'll be around 20% more efficient in low-band deployments and 50% more efficient in mid-band deployments (T-Mobile had numbers similar to that in some FCC filings during their merger proceedings, IIRC). Things like higher-order MIMO will help mid-band and higher applications more.
Given the chart of relative 5G/4G download speeds and the fact that T-Mobile usually beats their 4G network, it seems likely that AT&T is leaning heavily on LAA for their LTE performance and that's the difference-maker. As Release-16 becomes what networks and phones are using, that will likely change.
And it's important to note that right now, 5G isn't that much more efficient than LTE. Remember, when LTE came out, it was advertised as 5-8Mbps which we'd consider slow today. 5G NR will get better as it evolves. However, right now it looks like it'll be around 20% more efficient in low-band deployments and 50% more efficient in mid-band deployments (T-Mobile had numbers similar to that in some FCC filings during their merger proceedings, IIRC). Things like higher-order MIMO will help mid-band and higher applications more.
Given the chart of relative 5G/4G download speeds and the fact that T-Mobile usually beats their 4G network, it seems likely that AT&T is leaning heavily on LAA for their LTE performance and that's the difference-maker. As Release-16 becomes what networks and phones are using, that will likely change.
https://i.pcmag.com/imagery/fastest-networks/03efBScpy2I3SSV...
https://www.pcmag.com/fastest-mobile-networks/2020