Of course internal drills are very, very different from isometric exercises that you would be prescribed by a physical therapist, for instance, but any drill where there is no outside body movement neatly falls into the category of "isometric". The strict definition is met by the standing and partial squat drills done by every internal training group I have seen, for instance. My point is that Tomiki must have thought himself quite clever in selecting this explanation. It was the kind of statement that is both quite accurate and gives away nothing, and is therefore very typical of conversation on the topic.
After I had knee surgery, my physical therapy included working with a strength-and-conditioning coach who recommended isometric exercises as part of a regimen to help strengthen my knees. The prevailing western take regarding isometrics, like dynamic resistance training, is to focus on specific muscle groups, which is opposite of the IP/IS training, i.e. whole-body, approach without expressly engaging muscles. IP/IS training, in fact, has strengthened my knees noticeably beyond what I was able to achieve through conventional PT.
It's a bit of a well-worn declaration around these parts, but it's true: how many people do internal martial arts, knowing they're supposed to be doing whole-body conditioning in stillness and slow motion while eschewing muscle use, but can't handle the push of a 6'3", let alone 3'3" uke after several years or decades of training? (Nothing more ego-checking than to have a decent IP/IS practice session, only to go home and have your grade-school kids push up against your hara and immediately force you into stepping backward). So, I agree with Hugh that putting IP/IS training methodology into too big of a descriptive box doesn't help.
As for Kenji Tomiki, if he did associate Morihei Ueshiba's aiki with run-of-the-mill contemporary physical education, then that's where the thought of him talking in code about aiki as IP/IS gets a bit dodgy.
Also, a caveat: Howard's "two years" is not the same as just about everybody else's "two years", as has been described at length in the past.