How secure is Tesla "keyless" entry?

Tesla was the least stolen car last year.
There is no grey/black market for Telsa parts because every car gets serviced by Tesla.
Garage door openers are also easily targeted, but the local police department doesn't know of a single instance of a garage being burgled using a code breaking system.
In general, those with the skills and equipment to do so, can get a better paying job than car thief.
Professional thieves would use a flatbed--no key/fob/combination needed.

I realize this doesn't directly answer your question, but it's really a non-issue. The media likes to print this kind of story because it creates hits, and security companies like to write this kind of story because their income is based on people's fear. Creating fear is good for their business.

It's a grabbing headline, but here are a couple quotes even from the article:

It is becoming much harder to steal cars. According to the UK Office for National Statistics, car theft has fallen from 318,000 in 2002 to 77,500 last year.

But Ian Crowder, from motorists' group the AA, warned the risk should not be overstated. "By far the most common way of a car being stolen is still from thieves breaking into homes and stealing keys," he said.
And here's another article: Technology making car theft obsolete

Basically there will always be a back and forth between thieves and car companies, but cars like the Tesla are extremely hard to steal compared with older cars. And as Jerry pointed out there's not much reason for a thief to steal a Tesla, because there's not much they can do with it.