What does sympathetic and parasympathetic 'tone' mean?

Short answer
Parasympathetic tone is not necessarily mediated by tonic firing.

Background
Nice question! I think part of the confusion stems from terminology. Let's start with some definitions first.

• 'Tone' in sympathetic tone can be translated as mean activity level (Hayano & Yuda, 2019).
• 'Tonic' firing by neurons may be translated as a sustained response, which activates during the course of the stimulus (Wang et al., 2014), although some neurons have intrinsic tonic activity as well, like thalamocortical neurons. This opposes to phasic firing neurons.
• 'Phasic firing' refers to a transient response with one or few action potentials at the onset of stimulus followed by accommodation (Wang et al., 2014).

In your post you seem to assume that parasympathetic tone equals tonic firing activity. I'm not so sure about this. For example, I found a catching paper on the parasympathetic regulation of lung function in guinea pigs (which lacks sympathetic innervation altogether). The paper nicely illustrates (Fig. 1) that parasympathetic regulation of the bronchi in guinea pigs is mediated by both tonic and phasic (burst) stimulation (Myers, 2000):

[P]hasic neurons in guinea pig bronchial parasympathetic ganglia respond to a prolonged suprathreshold stimulation with a high‐frequency burst of action potentials ..., whereas tonic neurons are nonaccommodating and will respond with relatively lower frequency action potentials...

^{Fig. 1. Phasic and tonic neurons in guinea pig bronchial parasympathetic ganglia. source: (Myers, 2000)}

<sub>References
- Hayano & Yuda, J Physiol Anthropol (2019); 38(3) (2019)
- Myers, JCN (2000); 419(417): 439-50
- Wang et al., Channels (2014); 8(4): 298–307</sub>