I'm going to conclude this is almost certainly an error in §10.5.2 of the spec, and that the timer as the start event in an event sub-process is allowed.
- Tables 10.86 and 10.93 are both explicit in that the timer can be the trigger for an event sub-process.
- The non-interrupting timer start event is only useful in an event sub-process. That symbol would have no use if a timer event were not allowed to trigger an event sub-process.
- Section 10.5.6 consistently allows the use of the timer as the start event trigger
The issue was reported to OMG in 2010 (Issue 15532), although no further action was taken.
The same principle applies to Parallel Multiple events, which are similarly omitted from the same list in §10.5.2, but permitted in other sections.