Are the y-axis values informative when performing biolayer interferometry?
From the linked article:

We found that hACE2 bound to SARS-CoV-2 SB and SARS-CoV SB with respective equilibrium dissociation constants of 1.2 nM (Figure 2A) and 5.0 nM (Figure 2B).

An equilibrium dissociation constant (commonly called Kd) represents the propensity of a complex to dissociate into its constituent pieces.  A smaller value represents a smaller propensity to break apart, i.e. a stronger interaction.
So, yes, considering only the data presented, domain B from the SARS-CoV-2 spike protein shows a slightly higher affinity for hACE2 than the equivalent domain from SARS-CoV.  Note, however, that the authors are careful with their wording, and state in multiple places that the relative affinities of the homologous domains are comparable.

The SARS-CoV-2 SB engages human ACE2 (hACE2) with comparable affinity to SARS-CoV SB from viral isolates associated with the 2002–2003 epidemic (i.e., binding with high affinity to hACE2).

What's more telling are the binding constants (kon and koff) given in Table 1:

$$\begin{array}{c|c|c|} 
\text{} &amp; \text{SARS-CoV-2 S}^{B} &amp; \text{SARS-CoV S}^{B} \\ \hline
\text{K}_{D} \text{ (nM)} &amp; 1.2 ± 0.1 &amp; 5.0 ± 0.1 \\ \hline
\text{k}_{on} \text{ (M}^{-1} \text{s}^{-1}\text{)} &amp; 1.4 × 10^5  (2.3 ± 1.4 × 10^5) &amp; 1.4 × 10^5  (1.7 ± 0.7 × 10^5) \\ \hline
\text{k}_{off} \text{(s}^{-1}\text{)} &amp; 1.6 × 10^{−4}  (1.7 ± 0.8 × 10^{−4}) &amp; 7.1 × 10^{−4} (8.7 ± 5.1 × 10^{−4}) \\ \hline
\end{array}$$
Values reported represent the global fit to the data shown in Figures 2A and 2B and the averages obtained from five (SARS-CoV-2) or four (SARS-CoV) replicates carried out with different protein preparations are shown in parentheses.

Note that Kd is the reciprocal of the association constant, Ka, which is equal to the ratio of the binding constants.
$K_{a} = \frac{k_{on}}{k_{off}}$, $K_{d} = \frac{k_{off}}{k_{on}}$
Since the values given for kon derived from the global fit are the same for SARS-CoV-2 and SARS-CoV, the observed difference in Kd is due to the different values of koff.  So, at equilibrium, SARS-CoV-2 SB and SARS-CoV SB will associate with hACE2 at the same rate, though SARS-CoV SB dissociates faster.