Whilst it's true that the AR are mobilised as part of the CSF (Covid Support Force) the fact that they are taking over the Mobile Testing Units from regular units has nothing to do with the regulars being under strength.It's worse than that - my mate's son is in the PWRR TA and will be deployed on Covid testing in Kent on the 1st June, replacing a regular regiment. It has come to a sorry state when we are routinely mobilising and deploying Reserve units because the Regular Army is so under strength. God help us if another crisis blows up (as it has a nasty habit of doing).
The MOD mobilised 2K plus AR troops in readiness but in actual fact many of them have sat at home twindling their thumbs. By standing them up to cover the MTUs it allows the regular troops to get their missed leave and get back to normal training and duties.