The commonly stated assertion that spectrum is a limited resource is not quite accurate.
Spectrum is technically defined as a range of frequencies. That's all. In business parlance spectrum is also attached to large areas of land - this spatial dimension is more important than most people realize.
Radio signals themselves exist in space and time, not just in a range of frequencies. More radio signals - and data bandwidth - can be packed into a given space by shrinking the volume a given signal "occupies".
We can do this by reducing signal power and increasing cell density, in addition to other techniques described in the article. More cells, smaller cells. This is a big part of how 5G expands cell network capacity. But the telecoms have downplayed the effect of this relative to the the claim that spectrum is limited.
The mobile carriers have financial incentives to do this. These incentives are lower costs and monopoly control. Fewer bigger cells at higher power are cheaper than many smaller cells at lower power. The monopoly part is exclusive use of spectrum on a given piece of land.
The problem is, the legal attachment of spectrum works with very large areas of land (where km^2 is a smallish unit) and large periods of time (years), relative to radio signals. Both attachments are grossly inefficient.
By shrinking cell size (power) and increasing cell density, several orders of magnitude more network bandwidth is possible, plenty even to share (modulo cost of physical infrastructure).
Spectrum scarcity is a myth. The current legal regime enriches monopolists and is otherwise a tremendous waste of potential. We pay higher prices for unnecessarily limited bandwidth.